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                                                                   Exhibit 10.56


                                                       PERSONAL AND CONFIDENTIAL



                                  June 11, 1999



Mr. Thomas J. Bucknum
381 Beacon Street
Boston, MA 02116

Dear Tom:

This confirms your discussion with Jim Vincent and me related to your new role
in the organization. Effective June 10, 1999, your new title is Vice President
and General Counsel with a base salary of $225,000 and a target bonus of 30%.
Additionally, you have been granted 25,000 stock options as of June 10, 1999
with a grant value of $2,710,938 (25,000 shares at $108.4375). As an officer of
the company, you will be required to participate in our program for executive
stock ownership. We believe it is very important that our key leaders have
personal stock ownership. I will outline the program to you.

Additionally, in your new role you are now eligible for the following:

1.   VACATION - You are entitled to four (4) weeks vacation.

2.   SUPPLEMENTAL SAVINGS PLAN: You are entitled to participate in the
     Voluntary Executive Supplemental Savings Plan. This plan allows you to
     defer from your current  taxation up to 50% of your base salary and 100%
     of your 1999 bonus, if there is one. Rick Fisher can outline the details
     of the program if you are interested in participating.

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 PricewaterhouseCoopers.
     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


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Mr. Thomas J. Bucknum
June 11, 1999
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     "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., 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 twelve 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
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