EXHIBIT 99(d)(6)


                          IMCLONE SYSTEMS INCORPORATED
                                180 VARICK STREET
                            NEW YORK, NEW YORK 10014

                               September 19, 2001


Brian Markison
Senior Vice President-External Affairs
Bristol-Myers Squibb Company
345 Park Avenue
New York, New York 10154

Dear Brian:

         Bristol-Myers Squibb Company, a Delaware corporation ("BMS"), ImClone
Systems Incorporated, a Delaware corporation (the "Company") and Bristol-Myers
Squibb Biologics Company, a Delaware corporation and a wholly owned subsidiary
of BMS ("Acquisition Sub"), are entering into an Acquisition Agreement, dated
the date hereof (the "Acquisition Agreement"), pursuant to which BMS and
Acquisition Sub agree to make a partial tender offer (the "Offer") for
outstanding shares of common stock, par value $0.001 per share, of the Company,
at $70.00 per share net to the seller in cash (the "Offer Price"). I am a
stockholder of the Company and enter into this letter agreement, at BMS'
request, in connection with the Acquisition Agreement.

         Except as set forth herein, all capitalized terms used in this letter
agreement but not defined in this letter agreement shall have the meanings given
such terms in the Acquisition Agreement. For purposes of this letter agreement,
the term "Owned Shares" means the number of Shares of which I am the beneficial
owner on that date hereof, other than (i) any Shares that are owned by or held
for the benefit of any trust, foundation or other Person of which I am a trustee
or fiduciary and (ii) any Shares beneficially owned by me by virtue of Common
Stock Equivalents (as defined in the Stockholder Agreement) beneficially owned
by me.

         I confirm my agreement with you as follows:

         1. Except as provided in paragraph 2, I agree to validly tender (or to
cause the record owner of such Shares to validly tender), pursuant to and in
accordance with the terms of the Offer, a substantial portion of the Owned
Shares, and to not withdraw such Shares, except following termination of this
letter agreement pursuant to paragraph 5; PROVIDED, HOWEVER, that nothing in
this letter agreement shall require me to take any action which would create any
liability under Section 16(b) of the Exchange Act. I acknowledge and agree that
BMS's obligation to accept for payment and pay for such Shares is subject to the
terms and conditions of the Offer.

         2. Notwithstanding anything contained in this letter agreement, if the
board of directors of the Company changes its recommendation with respect to the
Offer or if




there is Third Party Change of Control Offer, the number of Shares required to
be validly tendered and not withdrawn in the Offer shall be limited to 100,000.

         3. I agree, subject to the proviso in paragraph 1, not to sell or
otherwise transfer or dispose of any of the Owned Shares, or any interest in any
of the Owned Shares, other than (i) pursuant to the Offer or (ii) with your
prior written consent, if such sale, transfer or disposition would prevent me
from performing my obligations under this letter agreement.

         4. Nothing herein shall be construed to require me, or any Person
controlled by me, to take any action or fail to take any action in violation of
any applicable law.

         5. This letter agreement shall terminate upon the earlier of (i)
acceptance for payment of the Shares validly tendered and not withdrawn in the
Offer, (ii) termination of the Acquisition Agreement pursuant to its terms,
(iii) termination of the Offer or (iv) the failure of BMS and Acquisition Sub to
commence the Offer in the time required by the Acquisition Agreement.

         6. This letter agreement will be governed by and construed in
accordance with the laws of the State of New York without giving effect to the
conflicts of laws principles that would otherwise apply thereunder.

         Please confirm that the foregoing correctly states the understanding
between us by signing and returning to me a counterpart hereof.

                                  Very truly yours,


                                  By: /s/ Samuel D. Waksal, Ph.D.
                                     -------------------------------
                                     Samuel D. Waksal, Ph.D.

Confirmed and agreed to on
the date first above written:

BRISTOL-MYERS SQUIBB COMPANY

By:      Brian Markison
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Title:   Senior Vice President -
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            External Affairs, Worldwide
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            Medicines Group