EXHIBIT 99.1

                  RISK FACTORS

You should carefully consider the following risk factors before you decide to
trade our common stock. Any of these risks could have a material adverse impact
on our business, financial condition, operating results or cash flows. This
could cause the trading price of our common stock to decline, and you may lose
part or all of your investment.

RISKS RELATED TO OUR BUSINESS

IF WE INCUR OPERATING LOSSES FOR LONGER THAN WE EXPECT, WE MAY BE UNABLE TO
CONTINUE OUR OPERATIONS.

From our inception through March 31, 2003, we have generated net losses totaling
$227 million. We expect to incur increasing and significant losses over the next
several years as we continue our clinical trials, apply for regulatory
approvals, continue development of our technologies, and expand our operations.
Phase III clinical trials are particularly expensive to conduct. We do not
expect to generate significant revenues for several years. To date, we have
generated product sales revenue from only one product, our feline leukemia
vaccine named Quilvax-FELV. Our revenues from Quilvax-FELV were $0.9 million for
the three months ended March 31, 2003. These revenues are generated through
sales of Quilvax-FELV to our marketing partner Virbac, S.A. This agreement
expired in July 2002 at which point we began to supply product to Virbac, S.A.
through month-to-month supply agreements. A long-term supply agreement is under
negotiation. If a long-term agreement is not executed, or if we cease to ship
them product on a month-to-month basis, we may not generate further revenues
from the sale of this product, the only product we currently sell. In addition,
any regulatory, marketing or other difficulties we experience with Quilvax-FELV,
could jeopardize that revenue stream.

IF WE FAIL TO OBTAIN THE CAPITAL NECESSARY TO FUND OUR OPERATIONS, WE WILL BE
UNABLE TO ADVANCE OUR DEVELOPMENT PROGRAMS AND COMPLETE OUR CLINICAL TRIALS.

On March 31, 2003, we had approximately $103.1 million in cash, cash equivalents
and short-term investments. In January 2003 we sold 6,250,000 shares of our
common stock, raising net proceeds of $59.5 million. We expect that we could
fund our development programs, clinical trials, and other operating expenses
into the third quarter of 2004. We plan to raise additional funds prior to that
time. Since our inception, we have financed our operations primarily through the
sale of equity. In order to finance our future operations, we will be required
to raise additional funds in the capital markets, through arrangements with
corporate partners, or from other sources. Additional financing, however, may
not be available on favorable terms or at all. If we are unable to raise
additional funds when we need them, we may be required to delay, reduce or
eliminate some or all of our development programs and some or all of our
clinical trials, including the development programs and clinical trials
supporting our lead cancer vaccine, Oncophage. We also may be forced to license
technologies to others that allocate to third parties substantial portions of
the potential value of these technologies.

WE MAY NOT RECEIVE SIGNIFICANT PAYMENTS FROM COLLABORATORS DUE TO UNSUCCESSFUL
RESULTS IN EXISTING COLLABORATIONS OR FAILURE TO ENTER INTO FUTURE
COLLABORATIONS.

Part of our strategy is to develop and commercialize some of our products by
continuing our existing collaborative arrangements with academic and corporate
collaborators and licensees and by entering into new collaborations. Our success
depends on our ability to successfully negotiate such agreements and on the
success of the other parties in performing research, preclinical and clinical
testing. Our collaborations involving QS-21, for example, depend on our partners
successfully completing clinical trials and obtaining regulatory approvals.
These activities frequently fail to produce marketable products. For example, in
March 2002, Elan Corporation and Wyeth Ayerst Laboratories announced a decision
to permanently cease dosing patients in their Phase IIA clinical trial of their
lead Alzheimer's vaccine containing our QS-21 adjuvant. Several of our
agreements also require us to transfer important rights to our collaborators and
licensees. These collaborators and licensees could choose not to devote
resources to these arrangements or, under certain circumstances, may terminate
these arrangements early. They may cease pursuing the program or elect to
collaborate with a different company. In addition, these collaborators and
licensees, outside of their arrangements with us, may develop technologies or
products that are competitive with those that we are developing. From time to
time we may also become involved in disputes with our collaborators. As a result
of these factors, our strategic collaborations may not yield revenues. In
addition, we may be unable to enter into new collaborations or enter into new
collaborations on favorable terms. Failure to generate significant revenue from
collaborations would increase our need to fund our operations through sales of
securities.

WE MUST RECEIVE SEPARATE REGULATORY APPROVALS FOR EACH OF OUR DRUGS AND VACCINES
IN EACH TYPE OF DISEASE BEFORE WE CAN MARKET AND SELL THEM IN THE UNITED STATES
OR INTERNATIONALLY, AND THIS APPROVAL PROCESS IS UNCERTAIN, TIME-CONSUMING AND
EXPENSIVE.

We and our collaborators cannot sell any drug or vaccine until it receives
regulatory approval from federal, state and local governmental authorities in
the United States, including the FDA, and from similar agencies in other
countries. Oncophage and any other drug candidate could take a significantly
longer time to gain regulatory approval than we expect or may never gain
approval. The process of obtaining and maintaining regulatory approvals for new
therapeutic products is lengthy, expensive and uncertain. It also can vary
substantially, based on the type, complexity and novelty of the product. Our
flagship product candidate, Oncophage, is a novel cancer therapeutic vaccine
that is personalized for each patient. To date, the FDA and foreign regulatory
agencies have approved only a limited number of cancer therapeutic vaccines for
commercial sale and have relatively little experience in reviewing personalized
medicine therapies. This lack of experience may lengthen the regulatory review
process for Oncophage, increase our development costs and delay or prevent
commercialization.



To obtain regulatory approvals, we must, among other requirements, complete
carefully controlled and well-designed clinical trials demonstrating that a
particular drug or vaccine is safe and effective for the applicable disease.
Several biotechnology companies have failed to obtain regulatory approvals
because regulatory agencies were not satisfied with the structure of clinical
trials or the ability to interpret the data from the trials; we could encounter
similar problems. The timing and success of a clinical trial is dependent on
enrolling sufficient patients in a timely manner, avoiding adverse patient
reactions, and demonstrating in a scientifically significant manner the efficacy
of a product. We rely on third party clinical investigators to conduct our
clinical trials and as a result, we may encounter delays outside our control.
Future clinical trials may not show that our drugs and vaccines are safe and
effective. In addition, we or the FDA might delay or halt the clinical trials,
including our Phase III trials of Oncophage, for various reasons, including:

     -   failure to comply with extensive FDA regulations;

     -   the product may not appear to be more effective than current therapies;

     -   the product may have unforeseen or significant adverse side effects or
         other safety issues;

     -   the time required to determine whether the product is effective may be
         longer than expected;

     -   we may be unable to adequately follow or evaluate patients after
         treatment with the product;

     -   patients may die during a clinical trial because their disease is too
         advanced or because they experience medical problems that may not be
         related to the product;

     -   sufficient numbers of patients may not enroll in our clinical trials;
         or

     -   we may be unable to produce sufficient quantities of the product to
         complete the trial.

Furthermore, regulatory authorities, including the FDA, may have varying
interpretations of our pre-clinical and clinical trial data, which could delay,
limit or prevent regulatory approval or clearance. Any delays or difficulties in
obtaining regulatory approval or clearances for our drugs or vaccines may:

     -   adversely affect the marketing of any products we or our collaborators
         develop;

     -   impose significant additional costs on us or our collaborators;

     -   diminish any competitive advantages that we or our collaborators may
         attain; and

     -   limit our ability to receive royalties and generate revenue and
         profits.

If we do not receive regulatory approval for our products in a timely manner, we
will not be able to commercialize them, and, therefore, our business and stock
price will suffer.

Even if we receive regulatory approval for our products, the FDA may impose
limitations on the indicated uses for which our products may be marketed. These
limitations could reduce the size of the potential market for that product.
Product approvals, once granted, may be withdrawn if problems occur after
initial marketing. Failure to comply with applicable FDA and other regulatory
requirements can result in, among other things, warning letters, fines,
injunctions, civil penalties, recall or seizure of products, total or partial
suspension of production, refusal of the government to renew our marketing
applications and criminal prosecution.

IF WE ARE UNABLE TO PURIFY HEAT SHOCK PROTEINS FROM SOME CANCER TYPES, THE SIZE
OF OUR POTENTIAL MARKET WOULD DECREASE.

Heat shock proteins occur naturally in the human body and activate powerful
cellular immune responses. Our ability to successfully commercialize Oncophage
for a particular cancer type depends on our ability to purify heat shock
proteins from that type of cancer. Based on our clinical trials conducted to
date, in renal cell carcinoma, we have been able to manufacture Oncophage from
93% of the tumors delivered to our manufacturing facility; for melanoma, 87%;
for colorectal cancer, 98%; for gastric cancer, 81%; for lymphoma, 89%; and for
pancreatic cancer, 30%. The relatively low rate for pancreatic cancer is due to
the abundance of proteases in pancreatic tissue. Proteases are enzymes that
break down proteins. These proteases may degrade the heat shock proteins during
the purification process. We have recently made process development advances
that have improved the manufacture of Oncophage from pancreatic tissue. In an
expanded Phase I pancreatic cancer study, Oncophage was manufactured from five
of five tumor samples (100%), bringing the aggregate success rate for this
cancer type to 46%.

We may encounter this problem or similar problems with other types of cancers as
we expand our research. If we cannot overcome these problems, the number of
cancer types that Oncophage could treat would be limited.

IF WE FAIL TO SUSTAIN AND FURTHER BUILD OUR INTELLECTUAL PROPERTY RIGHTS,
COMPETITORS WILL BE ABLE TO TAKE ADVANTAGE OF OUR RESEARCH AND DEVELOPMENT
EFFORTS TO DEVELOP COMPETING PRODUCTS.



If we are not able to protect our proprietary technology, trade secrets and
know-how, our competitors may use our inventions to develop competing products.
We currently have exclusive rights to 74 issued United States patents and 112
foreign patents. We also have rights to 60 pending United States patent
applications and 112 pending foreign patent applications. However, our patents
may not protect us against our competitors. The standards which the United
States Patent and Trademark Office uses to grant patents, and the standards
which courts use to interpret patents, are not always applied predictably or
uniformly and can change, particularly as new technologies develop.
Consequently, the level of protection, if any, that will be provided by our
patents if we attempt to enforce them and they are challenged in court, is
uncertain. In addition, the type and extent of patent claims that will be issued
to us in the future is uncertain. Any patents which are issued may not contain
claims which will permit us to stop competitors from using similar technology.

In addition to our patented technology, we also rely on unpatented technology,
trade secrets and confidential information. We may not be able to effectively
protect our rights to this technology or information. Other parties may
independently develop substantially equivalent information and techniques or
otherwise gain access to or disclose our technology. We generally require each
of our employees, consultants, collaborators, and certain contractors to execute
a confidentiality agreement at the commencement of an employment, consulting,
collaborative or contractual relationship with us. However, these agreements may
not provide effective protection of our technology or information or, in the
event of unauthorized use or disclosure, they may not provide adequate remedies.

WE MAY INCUR SUBSTANTIAL COSTS AS A RESULT OF LITIGATION OR OTHER PROCEEDINGS
RELATING TO PATENT AND OTHER INTELLECTUAL PROPERTY RIGHTS AND WE MAY BE UNABLE
TO PROTECT OUR RIGHTS TO, OR USE, OUR TECHNOLOGY.

If we choose to go to court to stop someone else from using the inventions
claimed in our patents, that individual or company has the right to ask the
court to rule that our patents are invalid and should not be enforced against
that third party. These lawsuits are expensive and would consume time and other
resources even if we were successful in stopping the infringement of our
patents. In addition, there is a risk that the court will decide that our
patents are not valid and that we do not have the right to stop the other party
from using the inventions. There is also the risk that, even if the validity of
our patents is upheld, the court will refuse to stop the other party on the
ground that such other party's activities are not covered by (that is, do not
infringe) our patents.

Furthermore, a third party may claim that we are using inventions covered by
such third party's patents and may go to court to stop us from engaging in our
normal operations and activities. These lawsuits are expensive and would consume
time and other resources. There is a risk that a court would decide that we are
infringing the third party's patents and would order us to stop the activities
covered by the patents. In addition, there is a risk that a court will order us
to pay the other party damages for having violated the other party's patents.
The biotechnology industry has produced a proliferation of patents, and it is
not always clear to industry participants, including us, which patents cover
various types of products. The coverage of patents is subject to interpretation
by the courts, and the interpretation is not always uniform. We know of patents
issued to third parties relating to heat shock proteins and alleviation of
symptoms of cancer, respectively. We have reviewed these patents, and we
believe, as to each claim in the patents, that we either do not infringe the
claim of the patents or that the claim is invalid. Moreover, patent holders
sometimes send communications to a number of companies in related fields,
suggesting possible infringement, and we, like a number of biotech companies,
have received this type of communication, including with respect to the third
party patents mentioned above. If we are sued for patent infringement, we would
need to demonstrate that our products either do not infringe the patent claims
of the relevant patent and/or that the patent claims are invalid, and we may not
be able to do this. Proving invalidity, in particular, is difficult since it
requires a showing of clear and convincing evidence to overcome the presumption
of validity enjoyed by issued patents. Additionally, one of the patent
applications licensed to us contains claims that are substantially the same as
claims in three of the third party patents mentioned above. The United States
Patent and Trademark Office has declared an interference proceeding with respect
to two of these third party patents to resolve this conflict. In an interference
proceeding, the party with the earliest effective filing date has certain
advantages. Although we believe that our claims have an earlier effective filing
date than the conflicting claims of the other patents, if this third party were
to prevail in the interference proceeding, it could result in abandonment of our
patent application and the potential need to seek a license from this party
which may not be available on reasonable terms, if at all.

Some of our competitors may be able to sustain the costs of complex patent
litigation more effectively than we can because they have substantially greater
resources. In addition, any uncertainties resulting from the initiation and
continuation of any litigation could have a material adverse effect on our
ability to continue our operations.

WE FACE LITIGATION THAT COULD RESULT IN SUBSTANTIAL DAMAGES AND MAY DIVERT
MANAGEMENT'S TIME AND ATTENTION FROM OUR BUSINESS.

Antigenics, our Chairman and Chief Executive Officer, Garo H. Armen, Ph.D., and
two brokerage firms that served as underwriters in our initial public offering
have been named as defendants in a civil class action lawsuit filed on November
5, 2001 in the Federal District Court in the Southern District of New York. Dr.
Armen was dismissed without prejudice from these claims in October 2002. Several
of plaintiff's claims against Antigenics were dismissed with leave to amend in
February 2003. For more detail regarding the status of the litigation as of this
Quarterly Report on Form 10-Q please see the description under Item 1, Legal
Proceedings.

The suit alleges that these underwriters charged secret excessive commissions to
certain of their customers in return for allocations of our stock in the
offering. The suit also alleges that shares of our stock were allocated to
certain of the underwriters' customers based upon an agreement by such customers
to purchase additional shares of our stock in the secondary market. We could be
required to pay substantial damages and, regardless of the outcome, the lawsuit
may cause a diversion of our management's time and attention from our business.

In addition, we may become involved in additional litigation with our commercial
partners or with others. Any such litigation could be expensive in terms of
out-of-pocket costs and management time, and the outcome of any such litigation
will be uncertain.



IF WE FAIL TO KEEP KEY MANAGEMENT AND SCIENTIFIC PERSONNEL, WE MAY BE UNABLE TO
SUCCESSFULLY DEVELOP OUR THERAPEUTIC DRUGS OR VACCINES, CONDUCT CLINICAL TRIALS
AND OBTAIN FINANCING.

We are highly dependent on our senior management and scientific personnel,
particularly Garo H. Armen, Ph.D., our chairman and chief executive officer,
Pramod K. Srivastava, Ph.D., our scientific founder, a member of our board of
directors and chairman of our scientific advisory board, Russell Herndon, our
president and chief operating officer, and Elma Hawkins, Ph.D., our vice
chairman. Since our manufacturing process is unique, our manufacturing and
quality control personnel are also very important. The competition for these and
other qualified personnel in the biotechnology field is intense. If we are not
able to attract and retain qualified scientific, technical and managerial
personnel, we may be unable to achieve our business objectives.

In addition, we have licensed a significant portion of our intellectual property
from institutions at which Dr. Srivastava has worked. We also sponsor research
in Dr. Srivastava's laboratory at the University of Connecticut Health Center in
exchange for the right to license discoveries made in that laboratory with our
funding. Dr. Srivastava is a member of the faculty of the University of
Connecticut School of Medicine. The regulations and policies of the University
of Connecticut Health Center govern the relationship between a faculty member
and a commercial enterprise. These regulations and policies prohibit Dr.
Srivastava from becoming our employee. Furthermore, the University of
Connecticut may modify these regulations and policies in the future to further
limit Dr. Srivastava's relationship with us. Dr. Srivastava has a consulting
agreement with us, which includes financial incentives for him to remain
associated with us, but that may not be enough to compel him to remain
associated with us even during the time covered by the consulting agreement. In
addition, this agreement does not restrict his ability to compete against us
after his association is terminated.

IF WE FAIL TO OBTAIN ADEQUATE LEVELS OF REIMBURSEMENT FOR OUR THERAPEUTIC DRUGS
OR VACCINES FROM THIRD PARTY PAYERS, THE COMMERCIAL POTENTIAL OF OUR THERAPEUTIC
DRUGS OR VACCINES WILL BE SIGNIFICANTLY LIMITED.

Our profitability will depend on the extent to which government authorities,
private health insurance providers and other organizations provide reimbursement
for the cost of our therapeutic drugs or vaccines. Many patients will not be
capable of paying for our therapeutic drugs or vaccines themselves. A primary
trend in the United States health care industry is toward cost containment.
Large private payers, managed care organizations, group purchasing organizations
and similar organizations are exerting increasing influence on decisions
regarding the use of particular treatments. Furthermore, many third party payers
limit reimbursement for newly approved health care products. Cost containment
measures may prevent us from becoming profitable.

PRODUCT LIABILITY AND OTHER CLAIMS AGAINST US MAY REDUCE DEMAND FOR OUR PRODUCTS
OR RESULT IN SUBSTANTIAL DAMAGES.

We face an inherent risk of product liability exposure related to testing our
therapeutic drugs or vaccines in human clinical trials and will face even
greater risks when we sell our drugs or vaccines commercially. An individual may
bring a product liability claim against us if one of our drugs or vaccines
causes, or merely appears to have caused, an injury. Product liability claims
may result in:

     -   decreased demand for our therapeutic drugs or vaccines;

     -   injury to our reputation;

     -   withdrawal of clinical trial volunteers;

     -   costs of related litigation; and

     -   substantial monetary awards to plaintiffs.

We manufacture Oncophage from a patient's tumor and a medical professional must
inject Oncophage into that same patient. A patient may sue us if we, a hospital
or a delivery company fails to deliver the removed tumor or that patient's
Oncophage. We anticipate that the logistics of shipping will become more complex
as the number of patients we treat increases, and it is possible that all
shipments will not be made without incident. In addition, administration of
Oncophage at a hospital poses another chance for delivery to the wrong patient.
Currently, we do not have insurance that covers loss of or damage to Oncophage
and do not know whether insurance will be available to us at a reasonable price
or at all.

We have limited product liability coverage for clinical research use of product
candidates. We also maintain limited product liability insurance for the
commercial sale of Quilvax-FELV. This limited insurance coverage may be
insufficient to fully compensate us for future claims.

WE MAY INCUR SIGNIFICANT COSTS COMPLYING WITH ENVIRONMENTAL LAWS AND
REGULATIONS.

We use hazardous, infectious and radioactive materials that could be dangerous
to human health, safety or the environment. We store these materials and various
wastes resulting from their use at our facility pending ultimate use and
disposal. We are subject to a variety of federal, state and local laws and
regulations governing the use, generation, manufacture, storage, handling and
disposal of these materials and wastes resulting from their use. We may incur
significant costs complying with both existing and future environmental laws and
regulations. In particular, we are subject to regulation by the Occupational
Safety and Health Administration and the Environmental Protection Agency and to
regulation under the Toxic Substances Control Act and the Resource Conservation
and Recovery Act. OSHA or the EPA may adopt regulations that may affect our
research and



development programs. We are unable to predict whether any agency will adopt any
regulations which could have a material adverse effect on our operations.

Although we believe our safety procedures for handling and disposing of these
materials comply with federal, state and local laws and regulations, we cannot
entirely eliminate the risk of accidental injury or contamination from these
materials. In the event of an accident, we could be held liable for any
resulting damages which could be substantial.

OUR COMPETITORS IN THE BIOTECHNOLOGY AND PHARMACEUTICAL INDUSTRIES MAY HAVE
SUPERIOR PRODUCTS, MANUFACTURING CAPABILITY OR MARKETING EXPERTISE.

Our business may fail because we face intense competition from major
pharmaceutical companies and specialized biotechnology companies engaged in the
development of therapeutic drugs or vaccines and other therapeutic products,
including heat shock proteins, directed at cancer, infectious diseases,
autoimmune disorders, and degenerative disorders. Several of these companies,
such as Dendreon, Stressgen, AVAX, Intracel and Cell Genesys, utilize similar
technologies and/or personalized medicine techniques. Additionally, many of our
competitors, including large pharmaceutical companies, have greater financial
and human resources and more experience. Our competitors may:

     -   commercialize their products sooner than we commercialize ours;

     -   develop safer or more effective therapeutic drugs or preventive
         vaccines and other therapeutic products;

     -   implement more effective approaches to sales and marketing;

     -   establish superior proprietary positions; or

     -   discover technologies that may result in medical insights or
         breakthroughs which may render our drugs or vaccines obsolete even
         before they generate any revenue.

More specifically, if we receive regulatory approvals, some of our therapeutic
drugs or vaccines will compete with well-established, FDA approved therapies
that have generated substantial sales over a number of years. We anticipate that
we will face increased competition in the future as new companies enter our
markets and scientific developments surrounding immunotherapy and other cancer
therapies continue to accelerate.

WE PLAN TO CONSOLIDATE OUR OPERATIONS IN A NEW FACILITY WHICH COULD CAUSE A
TEMPORARY DISRUPTION IN OUR BUSINESS.

We recently signed a lease for a facility in Lexington, Massachusetts. We intend
to consolidate our Woburn and Framingham operations into this facility in phases
over the next several years. The first phase, which we intend to complete during
2003, will involve the transfer of our Woburn manufacturing and administrative
operations to the Lexington facility. We expect that the build-out costs
associated with the first phase will be approximately $15 million. We do not
expect to initiate the build-out of the second phase, related to the Framingham
operations, until 2005. It is possible that our business operations could be
temporarily disrupted as a result of this facilities consolidation.

RISKS RELATED TO OUR STOCK

OUR OFFICERS AND DIRECTORS MAY BE ABLE TO BLOCK PROPOSALS FOR A CHANGE IN
CONTROL.

As of March 31, 2003, Antigenics Holdings L.L.C. controlled approximately 28% of
our outstanding common stock. Due to this concentration of ownership, Antigenics
Holdings may be able to prevail on all matters requiring a stockholder vote,
including:

     -   the election of directors;

     -   the amendment of our organizational documents; or

     -   the approval of a merger, sale of assets or other major corporate
         transaction.

Our directors and officers, if they elect to act together, can control
Antigenics Holdings. In addition, several of our directors and officers directly
and indirectly own shares of our common stock.

PROVISIONS IN OUR CHARTER DOCUMENTS COULD PREVENT OR FRUSTRATE ANY ATTEMPTS TO
REPLACE OUR CURRENT MANAGEMENT BY STOCKHOLDERS.

Our certificate of incorporation and bylaws contain provisions that could make
it more difficult for a third party to acquire us without consent of our board
of directors. Our certificate of incorporation provides for a staggered board
and removal of directors only for cause. Accordingly, stockholders may elect
only a minority of our board at any annual meeting, which may have the effect of
delaying or preventing changes in management. In addition, our certificate of
incorporation currently permits our board of directors to issue up to 25,000,000
shares of preferred stock and to determine the terms of those shares of stock
without any further action by our stockholders. Our issuance of preferred stock
could make it more difficult for a third party to acquire a majority of our
outstanding voting stock and thereby effect a change in the composition of our
board of directors. Our



certificate of incorporation also provides that our stockholders may not take
action by written consent. Our bylaws require advance notice of stockholder
proposals and nominations, and permit only our president or a majority of our
board of directors to call a special stockholder meeting. These provisions may
have the effect of preventing or hindering any attempts by our stockholders to
replace our current management. In addition, Delaware law also prohibits a
corporation from engaging in a business combination with any holder of 15% or
more of its capital stock until the holder has held the stock for three years
unless, among other possibilities, the board of directors approves the
transaction. The board may use this provision to prevent changes in our
management. Also, under applicable Delaware law, our board of directors may
adopt additional anti-takeover measures in the future.

OUR STOCK HAS LOW TRADING VOLUME AND OUR PUBLIC TRADING PRICE HAS BEEN VOLATILE.

Since our initial public offering on February 4, 2000, the per share price of
our common stock has fluctuated between $6.60 and $71.50 with an average daily
trading volume for the three months ended March 31, 2003 of approximately
343,000. The market has experienced significant price and volume fluctuations
that are often unrelated to the operating performance of individual companies.
In addition to general market volatility, many factors may have a significant
adverse effect on the market price of our stock, including:

     -   announcements of decisions made by public officials;

     -   results of our preclinical and clinical trials;

     -   announcements of technological innovations or new commercial products
         by us or our competitors;

     -   developments concerning proprietary rights, including patent and
         litigation matters;

     -   publicity regarding actual or potential results with respect to
         products under development by us or by our competitors;

     -   regulatory developments; and

     -   quarterly fluctuations in our revenues and other financial results.

THE SALE OF A SUBSTANTIAL NUMBER OF SHARES COULD CAUSE THE MARKET PRICE OF OUR
STOCK TO DECLINE.

The sale by us or the resale by stockholders of shares of our stock could cause
the market price of our stock to decline. As of March 31, 2003, we had
approximately 39,371,000 shares of common stock outstanding. All of these shares
are eligible for sale on the Nasdaq National Market, although certain of the
shares are subject to sale volume and other limitations.

We have filed registration statements to permit the sale of 5,236,831 shares of
common stock under our equity incentive plan and certain equity plans that we
assumed in the acquisitions of Aquila Biopharmaceuticals and Aronex
Pharmaceuticals. We have also filed a registration statement to permit the sale
of 300,000 shares of common stock under our employee stock purchase plan. As of
March 31, 2003, options to purchase approximately 4,703,000 shares of our stock
upon exercise of options with a weighted average exercise price per share of
$11.09 were outstanding. Many of these options are subject to vesting that
generally occurs over a period of up to five years following the date of grant.
As of March 31, 2003, warrants to purchase approximately 153,000 shares of our
common stock with a weighted average exercise price per share of $40.69 were
outstanding.