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                                                                   EXHIBIT 10.33

           Biogen, Inc. Voluntary Executive Supplemental Savings Plan

                                FOURTH AMENDMENT

     The Biogen, Inc. Voluntary Executive Supplemental Savings Plan ("Plan"), as
heretofore amended, is hereby further amended as follows:

1. Section 4.2 is amended in it entirety to read as follows:

     4.2  EMPLOYER CREDITS.

          (a)  AMOUNT OF MATCHING EMPLOYER CREDITS. For each calendar quarter
     (or a shorter period of time specified by the committee) during a plan
     year, each employer will credit a matching contribution amount to the
     account of each participant employed by such employer who makes matchable
     savings deposits during such calendar quarter (or such shorter period of
     time). The employer's matching contribution credits will be equal to 25% of
     the participant's matchable savings deposits during the calendar quarter
     (or such shorter period of time).

          (b)  TIME FOR MAKING EMPLOYER MATCHING CREDITS. The employer's
     matching amounts under subsection (a) will be credited to participants'
     accounts as soon as practicable after each calendar quarter (or such
     shorter period of time specified by the committee).

2. Section 5.1(c) is amended by inserting at the end of the first paragraph the
   parenthetical phrase "(other than the Biogen stock fund)", thereby not
   allowing participants to chose the Biogen stock fund as an investment option.

3. Section 5.1(c) is further amended by inserting at the end of the third
   paragraph and immediately after the phrase "calendar quarter" in the fourth
   paragraph the parenthetical phrase "(or such shorter period of time specified
   by the committee)".

4. Section 5.2(b) is amended in its entirety to read as follows: "A participant
   will have a fully vested interest in his employer matching credits account at
   all times."

5. Section 6.2 is amended in its entirety to read as follows: "Upon retirement
   from his employer, the participant will receive a single sum payment equal to
   his account balance, payable as soon as practicable after the committee's
   receipt of satisfactory evidence of the occurrence of his retirement."

6. Section 6.3(a) is amended by deleting the word "vested" each time it appears
   in the second sentence of said Section and by deleting the phrase "the end of
   the calendar quarter in which" in the third sentence of said Section.


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7. Section 6.4 is amended in its entirety to read as follows: "Upon a
   participant's termination of employment for any reason other than retirement
   or death, the participant will receive a single sum payment equal to his
   account balance as soon as practicable after the committee's receipt of
   satisfactory evidence of the termination of the participant's employment."

8. Except as expressly set forth herein, the Plan shall remain in full force and
   effect.


                                            BIOGEN, INC.

Date: December 13, 1999                     By:  /s/ Frank A. Burke, Jr.
                                               ---------------------------------
                                               Frank A. Burke, Jr.
                                               Vice President - Human Resources


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