Exhibit 4.2
             THE AMENDED 1992 REPLIGEN CORPORATION STOCK OPTION PLAN

     1. PURPOSE OF THE PLAN; COMPLIANCE WITH RULE 16B-3.

     (a) The 1992 Repligen Corporation Stock Option Plan, as amended (the
"Plan") is intended as an incentive to, and to encourage ownership of the stock
of Repligen Corporation, a Delaware corporation (the "Company") by, qualified
employees, outside directors and consultants of the Company and its
subsidiaries. It is intended that certain options granted thereunder will
qualify as incentive stock options (the "Incentive Stock Options") within the
meaning of Section 422 of the Internal Revenue Code of 1986, as amended (the
"Code") and that other options granted thereunder will not qualify as Incentive
Stock Options. Of such latter options, certain options (the "Non-Qualified
Options") will be granted in the sole discretion of the Committee referred to in
Section 4 hereof (the "Committee") and certain other options (the "Board
Options") will be granted to non-employee members of the Board of Directors of
the Company (the "Board of Directors") in accordance with the provisions of
Section 5 hereof.

     (b) With respect to persons subject to Section 16 of the Securities
Exchange Act of 1934, as amended (the "1934 Act"), transactions thereunder are
intended to comply with all applicable conditions of Rule 16b-3 or its
successors under the 1934 Act. To the extent that any provision of the Plan or
action of the Committee fails to so comply, it shall be deemed null and void, to
the extent permitted by law and (in the case of Incentive Stock Options and
Non-Qualified Options) deemed advisable by the Committee.

     2. COMMON STOCK SUBJECT TO THE PLAN.

     (a) Two million (2,000,000) shares of the authorized but unissued Common
Stock, $0.01 par value per share, of the Company have been allocated to the Plan
and will be reserved for issuance upon the exercise of options granted under the
Plan.

     (b) In addition, shares authorized and reserved for issuance under the
Company's 1982 Incentive Stock Option Plan, as amended, and 1987 Non-Statutory
Stock Option Plan (collectively, the "Prior Plans") which are not used
thereunder will also be reserved for issuance upon the exercise of options
granted under this Plan. If any option granted under either of the Prior Plans
shall expire or terminate for any reason without having been exercised in full,
the unpurchased shares subject thereto shall again be available for issuance
pursuant to the provisions of the Plan. The maximum number of shares of Common
Stock which may be issued in accordance with the provisions of this Section 2(b)
shall be 1,382,125.

     (c) The Company may, in its discretion, use shares held in the treasury in
lieu of authorized but unissued shares.

     3. ADMINISTRATION.

     The Plan shall be administered by the Committee; provided, that with
respect to the Board Options, the price, amount and timing of such options shall
be solely as set forth below:

     4. THE COMMITTEE; ISSUANCE OF INCENTIVE STOCK OPTIONS AND NON-QUALIFIED
        OPTIONS.

     (a) The Committee shall at all times be constituted to permit transactions
thereunder to comply with Rule 16b-3 under the 1934 Act, or any successor to
such Rule, which, as of the date hereof requires that it consist of two or more
members of the Board of Directors who do not receive Incentive Stock Options or
Non-Qualified Options and who do not receive, and have not at any time within
one year prior to their appointment as Committee members received, discretionary
grants or awards under any other stock plan of the Company or its subsidiaries.
The Committee shall be appointed by the Board of Directors, which may from time
to time appoint members of the Committee in substitution for members previously
appointed and may fill vacancies, however caused, in the Committee. The
Committee may select one of its members as its Chairman, and shall hold its
meetings at such times and places as it may determine. A majority of its members
(or both of its members, if there are only two) shall constitute a quorum. All
determinations of the Committee shall be


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made by a majority of its members, or, if there are only two members, by
unanimous vote. Any decision or determination reduced to writing and signed by a
majority (or if there are only two, both) of the members shall be fully as
effective as if it had been made by a majority (or unanimous, as the case may
be) vote at a meeting duly called and held. The Committee may appoint a
secretary, shall keep minutes of its meetings and shall make such rules and
regulations for the conduct of its business as it shall deem advisable.

     (b) Subject to the express provisions of the Plan, the Committee shall have
plenary authority, in its discretion, to determine the individuals to whom, and
the time or times at which, Incentive Stock Options and Non-Qualified Options
shall be granted, the number of shares to be subject to each such option, the
duration of each such option, the option price and method of payment and the
times or time within which (during the term of the option) all or portions of
each such option may be exercised. In making such determinations the Committee
may take into account the nature of the services rendered by the respective
individuals, their present and potential contributions to the Company's success
and such other factors as the Committee, in its discretion, shall deem relevant.
Subject to the express provisions of the Plan, the Committee shall also have
plenary authority to interpret the Plan to prescribe, amend and rescind rules
and regulations relating thereto, to determine the terms and provisions of the
respective stock option agreements in accordance with Section 9 hereof and to
make all other determinations necessary or advisable for the administration of
the Plan. The Committee's determinations on the matters referred to in this
Section 4(b) shall be conclusive. The Committee shall not have any discretionary
authority with respect to the award or terms and conditions of the Board
Options.

     5. ISSUANCE OF BOARD OPTIONS.

     Subject to the limitations set forth in this Section 5, (i) each
non-employee director of the Company, for so long as such person remains a
non-employee director of the Company, shall be entitled to receive an annual
option to purchase 10,000 shares of Common Stock to vest in full on the first
anniversary of the date of the grant, provided such person is still a director
on such anniversary and (ii) each person who becomes a member of the Board of
Directors who is not employed by the Company after the effective date of the
Plan shall receive, at the time such person first becomes a member of the Board
of Directors, an option to purchase 24,000 shares of Common Stock, vesting
equally over a three-year period from the date of grant. Notwithstanding
anything to the contrary contained herein, no person shall be entitled to
receive Board Options pursuant to the Plan covering more than an aggregate
100,000 shares. Those options granted to non-employee directors pursuant to this
section 5 are referred to herein as "Board Options."

     6. ELIGIBILITY.

     (a) Incentive Stock Options may be granted only to employees of the Company
or a subsidiary.

     (b) Non-Qualified Options may be granted only to (i) officers (who may also
be directors) and full or part-time employees employed on a salaried or
commission basis by the Company or its subsidiaries, (ii) members of the Board
of Directors of the Company who are not included within the group of individuals
referenced in the foregoing clause, and (iii) any individual performing services
for the Company or any subsidiary as an independent contractor pursuant to a
written or oral agreement with the Company or a subsidiary.

     (c) Board Options may be granted only to members of the Board of Directors
of the Company who are not employees of either the Company or its subsidiaries.

     (d) For purposes of the Plan, the term "subsidiary "shall mean any
corporation (other than the Company) in an unbroken chain of corporations
beginning with the Company if, at the time of the granting of the option, each
of the corporations other than the last corporation in the unbroken chain owns
stock possessing 50% or more of the total combined voting power of all classes
of stock in one of the other corporations in such chain, or such other meaning
as may be hereafter ascribed to it in Section 424 of the Code.


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     7. OPTION PRICES.

     (a) The purchase price of the Common Stock under each Incentive Stock
Option shall not be less than 100% of the fair market value of the stock at the
time of the granting of the option. For purposes hereof, fair market value shall
be equal to the NASDAQ National Market closing price (or the closing price on an
exchange if the Common Stock is then traded on an exchange) per share of Common
Stock for the day as of which an Incentive Stock Option is granted, or such
other amounts (which may be different amounts as between the various types of
options) as shall be determined from time to time in good faith by the Committee
pursuant to such criteria as it may determine to be appropriate.

     (b) The purchase price of the Common Stock under each Non-Qualified Option
shall not be less than the par value of the Common Stock.

     (c) The purchase price of the Common Stock under each Board Option shall be
equal to the NASDAQ National Market closing price (or the closing price on an
exchange if the Common Stock is then traded on an exchange) per share of Common
Stock for the date of the grant of such Board Option.

     (d) Incentive Stock Options and Non-Qualified Options may be exercised, in
the discretion of the Committee, by payment of the option price in full (i) in
cash, (ii) by surrender of shares of the capital stock of the Company having a
fair market value equal to the option price on the date of exercise, or (iii)
any combination of the foregoing. Board Options may be exercised by payment of
the option price in full by means of any of the three methods of payment
specified in the preceding sentence.

     (e) The proceeds of sale of stock subject to option are to be added to the
general funds of the Company or to the shares of the Common Stock of the Company
held in its treasury, and used for its corporate purposes as the Board of
Directors shall determine.

     (f) Notwithstanding any provision herein to the contrary, no Incentive
Stock Option shall be granted to any individual whose ownership of Common Stock
of the Company or one of its subsidiaries exceeds the limitations set forth in
Section 422(b)(6) of the Code unless such option price is at least 110% of the
fair market value of the stock at the time of the granting of the option.

     8. OPTION AMOUNTS.

     The maximum aggregate fair market value (determined at the time an option
is granted in the same manner as provided for in Section 7(a) hereof) of the
Common Stock of the Company with respect to which Incentive Stock Options are
exercisable for the first time by any optionee during any calendar year (under
all plans of the Company and its subsidiaries) shall not exceed $100,000.

     9. FORM OF OPTIONS.

     (a) Incentive Stock Options, Non-Qualified Options and Board Options shall
be in such form conforming to applicable legal requirements as shall be approved
from time to time by the Committee. The form of any such options may vary among
optionees.

     10. TERM OF OPTIONS; EXERCISE OF OPTIONS.

     (a) The term of each option shall be not more than ten (10) years from the
date of granting thereof, provided that no Incentive Stock Option shall be
granted to any individual whose ownership of Common Stock of the Company or its
subsidiaries exceeds the limitations set forth in Section 422(b)(6) of the Code
unless the term of his or her option does not exceed a period of five (5) years
from the date of the grant, or such shorter period as is prescribed in Section
10 hereof.

     (b) Within the limits specified in Section 10(a) hereof, Incentive Stock
Options and Non-Qualified Options will be exercisable at such time or times, and
subject to such restrictions and conditions, as the


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Committee shall, in each instance, approve, which need not be uniform for all
optionees; provided, however, that except as provided in Sections 11 and 12
hereof, no such option granted to an employee of the Company or a subsidiary may
be exercised at any time unless the optionee is then an employee of the Company
or a subsidiary and has been so employed continuously since the granting of the
option.

     (c) Notwithstanding any provision to the contrary contained herein, upon
the removal or resignation from the Board of Directors of a holder of a Board
Option, such holder may exercise such Board Option within three (3) months of
his resignation or removal (but in any case not after ten (10) years from the
date of the granting of the option) to the same extent that he was entitled to
exercise it as of the date of such resignation or removal.

     (d) The holder of an option shall have none of the rights of a stockholder
with respect to the shares subject to option until such shares shall be issued
to him upon the exercise of his option.

     (e) With respect to persons subject to Section 16 under the 1934 Act,
options granted thereunder must be held by the optionee for at least six (6)
months from the date of grant to the date of disposition of the option (other
than by exercise) or the Common stock underlying such option.

     11. TERMINATION OF EMPLOYMENT.

     (a) Any employee of the Company or a subsidiary who has been issued an
option thereunder must exercise the option prior to his termination of
employment, except that if an employee terminates his employment voluntarily,
such employee shall be permitted to exercise any such option then held by him at
any time within three (3) months after such termination (but in any case not
after ten (10) years from the date of the granting thereof) to the same extent
that he was entitled to exercise it at the date of such termination of
employment.

     (b) If the holder of an Incentive Stock Option or a Non-Qualified Option
terminates employment on account of disability he may exercise such option to
the extent he was entitled to exercise it at the date of such termination at any
time within one (1) year of the termination of his employment (but in any case
not after ten (10) years from the date of the granting thereof). For this
purpose a person shall be deemed to be disabled if he is permanently and totally
disabled within the meaning of Section 422(c)(6) of the Code, which, as of the
date hereof, shall mean that he is unable to engage in any substantial gainful
activity by reason of any medically determined physical or mental impairment
which can be expected to result in death or which has lasted or can be expected
to last for a period of not less than 12 months. A person shall be considered
disabled only if he furnishes such proof of disability as the Committee may
require.

     (c) If the holder of an Incentive Stock Option or Non-Qualified Option
issued thereunder retires on or after the attainment of age sixty-five (65),
such options may be exercised as determined by the Committee but in no event
more than ten (10) years after the date of granting thereof.

     (d) If the employment of the holder of an Incentive Stock Option or
Non-Qualified Option is terminated for "cause," all such options not yet
exercised shall be exercisable within seven (7) days of such termination, and
shall thereafter cease to be of any further force or effect. For purposes of the
foregoing,"cause" shall have the meaning set forth from time to time in the
employee handbook generally distributed by the Company to its employees.

     (e) Incentive Stock Option and Non-Qualified Options granted under the Plan
shall not be affected by any change of employment so long as the holder thereof
continues to be an employee of the Company or a subsidiary of the Company. The
option agreements relating to Incentive Stock Options and Non-Qualified Options
may contain such provisions as the Committee shall approve with reference to the
effect of approved leaves of absence. Nothing in the Plan or in any Incentive
Stock Option or Non-Qualified Option granted pursuant to the Plan shall confer
on any individual any right to continue in the employ of the Company or a
subsidiary of the Company or interfere in any way with the right of the Company
or a subsidiary of the Company to terminate such individual's employment at any
time.


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     12. DEATH OF HOLDER OF OPTION.

     In the event of the death of an individual to whom an Incentive Stock
Option or Non-Qualified Option has been granted under the Plan, while he is
employed by the Company (or a subsidiary of the Company) or within three (3)
months after the termination of his employment (or one (1) year in the case of
the termination of employment of an option holder who is disabled as above
provided), the Incentive Stock Option or Non-Qualified Option heretofore granted
to him may be exercised, to the extent that he was entitled to exercise it at
the date of such death, by a legatee or legatees of the option holder under his
last will, or by his personal representatives or distributees, at any time
within a period of two (2) years after his death (but in any case not after ten
(10) years from the date of granting thereof), and only if and to the extent
that he was entitled to exercise the option at the date of his death.

     13. NON-TRANSFERABILITY OF OPTIONS.

     Each option granted under the Plan shall, by its terms, be non-transferable
otherwise than by will or the laws of descentand distribution and an option may
be exercised, during the lifetime of the holder thereof, only by him.

     14. SUCCESSIVE OPTION GRANTS.

     Successive option grants may be made to any holder of options under the
Plan.

     15. ADJUSTMENTS UPON CHANGES IN CAPITALIZATION OR CORPORATE ACQUISITIONS.

     (a) In the event of a consolidation or merger of the Company with another
corporation, or the sale or exchange of all or substantially all of the assets
of the Company, or a separation, reorganization or liquidation of the Company,
each holder of an outstanding option issued in accordance herewith shall be
entitled to receive upon exercise and payment in accordance with the option's
terms the same shares, securities or property as he would have been entitled to
receive upon the occurrence of such event if he had been, immediately prior to
such event, the holder of the number of shares of Common Stock purchasable under
his option, or, if another corporation shall be the survivor, such corporation
shall substitute therefor substantially equivalent shares, securities or
property of such other corporation; provided, however, in lieu of the foregoing
the Committee may upon written notice to each holder of an outstanding Incentive
Stock Option or Non-Qualified Option provide that such option (but not a Board
Option) shall terminate on a date not less than 20 days after the date of such
notice unless theretofore exercised. In connection with such notice, the
Committee may in its discretion accelerate or waive any deferred exercise
period.

     (b) In the event the Company or a subsidiary of the Company enters into a
transaction described in Section 424(a) of the Code with any other corporation,
the Committee may grant options to employees or former employees of such
corporation in substitution of options previously granted to them upon such
terms and conditions as shall be necessary to qualify such grant as a
substitution described in Section 424(a) of the Code.

     (c) The number of shares of Common Stock for which options may be granted
thereunder and any references to specific amounts of shares (including but not
limited to those references set forth in Section 5 hereof) shall be
appropriately adjusted if the number of outstanding shares of Common Stock of
the Company is increased or reduced by split-up, reclassification, stock
dividend or the like. The number of shares previously optioned thereunder and
not theretofore delivered and the option price per share shall likewise be
adjusted whenever the number of outstanding shares of Common Stock is increased
or reduced by any such procedure.

     16. AMENDMENT AND TERMINATION.

     (a) The Board of Directors may at any time terminate the Plan, or make such
modifications of the Plan as it shall deem advisable; provided, however, that
the Board of Directors may not, without further approval


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by the holders of Common Stock: increase the maximum number of shares as to
which options may be granted under the Plan (except under the anti-dilution
provisions contained in Section 15 hereof); change the class of persons to whom
options may be granted; withdraw the authority to administer the Plan(insofar as
it relates to Incentive Stock Options and Non-Qualified Options) from the
Committee; extend the duration of the Plan; provide for any discretion to be
vested in the Committee or any other entity with respect to the award of the
Board Options; or materially increase the benefits accruing to persons subject
to Section 16 of the 1934 Act. No termination or amendment of the Plan may,
without the consent of the optionee to whom any option shall theretofore have
been granted, adversely affect the rights of such optionee under such option.

     (b) The provisions of this Plan relating to the Board Options shall not be
amended more than once every six months, other than to comport with changes in
the Code, the Employee Retirement Income Security Act of 1974, as amended, or
the respective rules thereunder.

     17. EFFECTIVENESS OF THE PLAN.

     The Plan shall become effective upon adoption by the Board of Directors
subject, however, to its further approval by the stockholders of the Company
given within twelve months of the date the Plan is adopted by the Board of
Directors at a regular meeting of the stockholders or at a special meeting duly
called and held for such purpose. Grants of options may be made prior to such
stockholder approval but all option grants made prior to stockholder approval
shall be subject to the obtaining of such approval and if such approval is not
obtained, such options shall not be effective for any purpose.

     18. WITHHOLDING OF APPLICABLE TAXES.

     The Company shall be entitled to withhold the amount of any tax
attributable to any shares deliverable under this Plan after giving the person
entitled to receive such shares notice as far in advance as practicable, and the
Company may defer making delivery if any such tax may be pending unless and
until indemnified to its satisfaction. Alternatively, the Company shall have the
right to reduce the number of shares otherwise required to be delivered upon
exercise of an option granted thereunder by an amount which would have a fair
value on the date of such exercise equal to all taxes required to be withheld by
the Company with respect to such exercise. In connection with such withholding,
the Company may make any such arrangements as are consistent with this Plan as
it may deem appropriate.

     19. TIME OF GRANTING OF OPTIONS.

     (a) A grant of an Incentive Stock Option or Non-Qualified Option under the
Plan shall be deemed to be made on the date on which the Committee, by formal
action of its members duly recorded in the records thereof, makes an award of an
option (but in no event prior to the adoption of the Plan by the Board of
Directors); provided, that such option is evidenced by a written option
agreement duly executed on behalf of the Company and on behalf of the optionee
within a reasonable time after the date of the Committee action.

     (b) A grant of a Board Option shall be deemed to be made on the applicable
dates provided in Section 5 hereof.

     20. TERM OF PLAN.

     This Plan shall terminate ten (10) years after the date on which it is
approved and adopted by the Board of Directors and no option shall be granted
thereunder after the expiration of such ten-year period. Options outstanding at
the termination of the Plan shall continue in full force and effect and shall
not be affected thereby.

                                      * * *

     The foregoing Plan was approved and adopted by the Board of Directors of
the Company on April 17, 1992. Amendments were approved on April 14, 1994,
September 10, 1996 and on September 14, 2000.


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