[BIOGEN, INC. LETTERHEAD]

                                                       PERSONAL AND CONFIDENTIAL

                                            April 2, 1996



Burt Adelman, M.D.
Vice President, Regulatory Affairs
Biogen Inc.
14 Cambridge Center
Cambridge, MA 02142

Dear Burt:

To confirm our recent discussions, your employment letter is hereby amended to
include the following additional provisions, in recognition of your position as
Vice President, Regulatory Affairs and an executive officer of Biogen.

1.   PERFORMANCE BONUS: Your performance bonus target is 25%.

2.   SUPPLEMENTAL SAVINGS PLAN: You are eligible to participate in the Voluntary
     Executive Supplemental Savings Plan.

3.   LIFE INSURANCE: You will be provided Biogen's Executive Term Life Insurance
     coverage for a total of $1,000,000. This coverage is based on your
     successfully meeting the medical standards as stated in the Executive Term
     insurance policy.

4.   TAX REVIEW/PREPARATION: You are entitled to the preparation and/or review,
     including review of estimated taxes of your annual Federal and State tax
     returns, which is currently administered through Price Waterhouse. The cost
     of this service is covered by Biogen.

5.   INVOLUNTARY TERMINATION: If you are involuntarily terminated from
     employment with Biogen (other than for cause), Biogen will protect you by
     paying you a supplementary amount (the "Supplementary Amount") equal to
     your then present BASE salary for a period (the "Extra Period") ending on
     the earlier of (i) the date twelve months from your termination and (ii)
     the date you start another job. During such period, Biogen will also pay to
     continue your health benefits (i.e.,


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April 2, 1996
Burt Adelman, M.D.
Page 2

     health and dental plan coverage), provided such benefits are accorded
     employees generally and Biogen can obtain the relevant coverage. If you
     need continued coverage to prevent a gap in health coverage between your
     Biogen coverage and that at your new job, Biogen will extend such coverage
     for up to 30 days (to the extent that the Extra Period is less than 12
     months) after you start your new job. The Supplementary Amount will be paid
     on the same schedule as your salary would have been paid. You will not be
     an employee of Biogen during the time of such payments and will not accrue
     any benefits or other rights (such as, but not limited to, pension plan
     vesting or accrual, stock option vesting, vacation pay, etc.) during such
     period except health benefits as described above. You agree to notify us
     when you accept a new job.

                                             Sincerely,

                                             /s/ Frank A. Burke, Jr.

                                             Frank A. Burke, Jr.
                                             Vice President, Human Resources


cc:    Personnel File

Source: OneCLE Business Contracts.