1
                                                                   EXHIBIT 10.28


                                CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN
                                THIS DOCUMENT, MARKED BY BRACKETS, IS FILED WITH
                                THE SECURITIES AND EXCHANGE COMMISSION PURSUANT
                                TO RULE 24b-2 OF THE SECURITIES EXCHANGE ACT OF
                                1934, AS AMENDED.

COR  COR THERAPEUTICS, INC.                           256 E. Grand Avenue
via fax 011-32-2-264-2790 and Federal Express         South San Francisco
                                                      California 94080
                                                      415-244-6800
                                                      Fax 415-244-9208



May 16, 1996

Mr. Alfred Hoffait
Solvay S.A.
rue de Ransbeek 310
B-1120 Brussels BELGIUM

Dear Mr. Hoffait:

As you are aware, COR is in the process of locating and qualifying possible
second sources for the supply of Integrilin(TM).

COR notes that our License and Supply Agreement, dated 27 July 1994, includes a
list of possible secondary sources and provides under Article 2.2 for additional
companies to be added to the list from time to time upon the agreement of COR
and Solvay.

To that end, COR would like to amend Exhibit C of our License and Supply
Agreement to include the following companies in addition to those already
listed:

                                       [*]

Thank you for your attention to this matter. To confirm your acknowledgment and
agreement to this addition, could you please sign both original copies of this
letter and return one to COR (attn: Jayne Lange) for our records.

Sincerely,

Mark D. Perrin
Executive Vice President
Commercial Operations


Acknowledged and Agreed:

For Solvay SA:

- -------------------------
Signed

- -------------------------
Name
June 6, 1996
- -------------------------
Date




                                         CONFIDENTIAL TREATMENT REQUESTED = [*]