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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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                                                                   EXHIBIT 10.35

                                  BIOGEN, INC.
                     Supplemental Executive Retirement Plan

                    FIRST AMENDMENT TO THE 4/1/96 RESTATEMENT

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

Section 3.4 is inserted and shall state:

     "An individual is not considered a Participant for purposes of this Plan if
     the individual is classified as a consultant or contractor under the
     Company's or the Company's affiliate's regular personnel classifications
     and practices, or he is a party to an agreement to provide services to the
     Company or the Company's affiliate without participating in the Plan,
     notwithstanding that such individual may be treated as a common law
     employee for payroll tax or other legal purposes."

Date:  September 27, 1999                   BIOGEN, INC.

                                            By: /s/ Frank A. Burke, Jr.
                                               ---------------------------------
                                                Frank A. Burke, Jr.
                                                Vice President - Human Resources<PAGE>   1
                                                                   EXHIBIT 10.39

             Biogen, Inc. Voluntary Board of Directors Savings Plan

                                 THIRD AMENDMENT

     The Biogen, Inc. Voluntary Board of Directors Savings Plan, as heretofore
amended (the "Plan"), is hereby further amended as follows:

Section 6.4 is amended in its entirety to read as follows:

     6.4  INSTALLMENT DISTRIBUTIONS IN CERTAIN CASES.

          (a)  PARTICIPANT. Notwithstanding the provisions of Section 6.2, 6.3
     and 6.4, a participant may, at the time of filing an enrollment form under
     Section 4.1(b), designate that the amount payable to him hereunder will be
     paid in a number (minimum of two and maximum of ten) of annual installment
     payments, as specified by the participant.

          (b)  BENEFICIARY. A participant may designate that, if the participant
     dies before receiving the entire amount payable to him hereunder, the
     beneficiary will receive either:

               (i)  a number of annual installment payments equal to

                    (A)  the number the participant elected for himself under
                         subsection (a) above (if the participant dies before
                         receiving any installment payments), or

                    (B)  the number of remaining installment payments due to the
                         participant under subsection (a) above (if the
                         participant dies after receiving one or more
                         installment payments); or

               (ii) a single payment.

     Payment to the beneficiary will commence as provided herein as soon as
     practicable after the committee's receipt of satisfactory evidence of the
     death of the participant.

          If the participant fails to designate the form of payment to the
     beneficiary, the default form will be installments under (i) above. If
     installment payments are payable to the beneficiary, with the consent of
     the committee, a participant may subsequently change the form of payment to
     his beneficiary (but not the form of

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     payment to himself under Section 6.2 or 6.4) to a single payment by filing
     a written instrument so specifying with the committee.

          (c)  INSTALLMENT PAYMENTS. Where installment payments are due, the
     first annual installment payment will be paid out on the date specified in
     Section 6.2, 6.3 or 6.4 (whichever is applicable) and subsequent annual
     installments will be paid on succeeding anniversaries of the first payment
     date. The amount of each annual installment payment will be determined by
     multiplying the vested amount in the participant's account by a fraction
     whose numerator is one and whose denominator is the number of remaining
     annual installment payments.

          (d)  DEATH OF BENEFICIARY. If a participant's designated beneficiary
     is receiving installment payments and dies before receiving payment of all
     the annual installments, the designated beneficiary's estate will receive a
     lump-sum payment of the amount remaining to be distributed to such deceased
     beneficiary. Such payment will be made as soon as practicable after the
     committee's receipt of satisfactory evidence of the death of the designated
     beneficiary.

                                            BIOGEN, INC.

Date: September 27, 1999                    By:  /s/ Frank A. Burke, Jr.
                                               ---------------------------------
                                                Frank A. Burke, Jr.
                                                Vice President - Human Resources

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                                                                   EXHIBIT 10.40

             Biogen, Inc. Voluntary Board of Directors Savings Plan

                                FOURTH AMENDMENT

     The Biogen, Inc. Voluntary Board of Directors Savings Plan ("Plan"), as
heretofore amended, is hereby further amended as follows:

1.   Section 5.1(b) 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.

2.   Section 6.2(a) is amended by deleting the phrase "the end of the calendar
     quarter in which" in the second sentence of said Section.

3.   Section 6.3 is amended by deleting the second sentence of said Section in
     its entirety and substituting in place thereof the following sentence:
     "Distribution will be made in a single lump sum payment as soon as
     practicable after the committee's receipt of satisfactory evidence of the
     occurrence of the event causing distribution."

4.   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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