AMENDMENT NUMBER ONE
                                     TO THE
                 RIGHTS AGREEMENT DATED AS OF SEPTEMBER 16, 1996
                         BETWEEN CELGENE CORPORATION AND
            AMERICAN STOCK TRANSFER & TRUST COMPANY, AS RIGHTS AGENT


Reference is made to the Rights Agreement dated as of September 16, 1996 between
Celgene Corporation and American Stock Transfer & Trust Company, as Rights Agent
(the "Rights  Agreement").  Capitalized terms not otherwise defined herein shall
have the meaning ascribed thereto in the Rights Agreement.

1.   Section  7(a) of the Rights  Agreement  is deleted and  restated to read as
     follows:

          "Section 7. EXERCISE OF RIGHTS;  PURCHASE  PRICE;  EXPIRATION  DATE OF
RIGHTS.  (a) Subject to Section 7(e) hereof,  the registered holder of any Right
Certificate  may  exercise  the Rights  evidenced  thereby  (except as otherwise
provided  herein)  in whole or in part at any time after the  Distribution  Date
upon surrender of the Right  Certificate,  with the form of election to purchase
and the  certificate  on the reverse side thereof duly  executed,  to the Rights
Agent at the principal office or offices of the Rights Agent designated for such
purpose,  together  with payment of the aggregate  Purchase  Price for the total
number of Common  Shares (or other  securities,  as the case may be) as to which
such  surrendered  Rights are exercised,  at or prior to the earliest of (i) the
close of business on February 17, 2010 (the "Final  Expiration  Date"),  or (ii)
the time at which the Rights are  redeemed as provided in Section 23 hereof (the
"Redemption Date")."

2.   Section  7(b) of the Rights  Agreement  is deleted and  restated to read as
     follows:

          "(b) The purchase price (the "Purchase Price") per whole Common Share
at which a holder of Rights may purchase Common Shares or (subject to Section 14
hereof)  fractions  thereof  upon  exercise of such Rights  shall  initially  be
$700.00,  shall be  subject  to  adjustment  from  time to time as  provided  in
Sections 11 and 13(a) hereof,  and shall be payable in accordance with paragraph
(c) below."


                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK.]







Except as amended hereby,  the Rights Agreement shall continue in full force and
effect.

Dated as of February 18, 2000

CELGENE CORPORATION


By: /s/ Robert J. Hugin
    Name: Robert J. Hugin
    Title: Senior Vice President and Chief Financial Officer

Countersigned:

AMERICAN STOCK TRANSFER & TRUST COMPANY



By: /s/ Herbert J. Lemmer
    Name: Herbert J. Lemmer
    Title: Vice President