Document:

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                                                                   Exhibit 10.50

                                  March 10, 2000

Mr. James L. Vincent
7 Audubon Road
Weston, MA 02193

Dear Jim:

         This letter will serve as notice that Biogen, Inc. would like to extend
your chairmanship of the Board of Directors for an additional three years under
the terms of the letter agreement dated November 21, 1996 (the "Letter
Agreement") amending your Employment Agreement dated as of September 23, 1985
(the "Employment Agreement") such that your chairmanship will continue after the
end of the Initial Chairmanship Term, as defined in the Letter Agreement, on
February 14, 2001 until February 14, 2004 (the "Extended Term") unless earlier
terminated by your resignation or earlier removal.

         During the Extended Term, after appointment of a successor Chief
Executive Officer, your duties as Chairman will once again consist of those
activities specified under Section 1 of the Letter Agreement.

         All other terms of the Letter Agreement and the Employment Agreement,
as amended by the Letter Agreement and this letter, shall apply during the
Extended Term.

         If you agree with extension of your chairmanship on the above terms,
please execute this letter and the enclosed copy in the space provided below.

                                      Sincerely,

                                      Biogen, Inc.

                                         By:      /s/ Roger H. Morley
                                                  Chairman, Compensation and
                                                  Management Resources Committee

                                         By:      /s/ Harold W. Buirkle
                                                  Member, Compensation and
                                                  Management Resources Committee
                                         By:      /s/ Phillip A. Sharp
                                                  Member, Compensation and
                                                  Management Resources Committee

<PAGE>   2

                                         By:      /s/ Mary L. Good
                                                  Member, Compensation and
                                                  Management Resources Committee

Acknowledged and agreed

/s/ James L. Vincent<PAGE>   1

                                                                   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

<PAGE>   2

Mr. Thomas J. Bucknum
June 11, 1999
Page 2

     "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
FAB:bk<PAGE>   1

                                                                   Exhibit 10.57
July 19, 1999

Mr. Kees Been
121 Whitebridge Meadows Lane
St. Louis, MO 63141

Dear Kees:

This letter supersedes my previous letter to you dated July 9, 1999 and
represents Jim Vincent's amended offer to you on behalf of Biogen. You are
offered the position of Vice President, Business Development at an annual salary
of $250,000. You will report directly to Jim Vincent. Consistent with Biogen's
compensation policy, you will be eligible for a pro-rated merit salary review at
year-end 1999 and annually thereafter. You will be eligible to participate in
Biogen's annual incentive compensation plan with a 30% target bonus beginning in
the year ending December 31, 2000.

Upon employment you will receive $200,000 as a one-time special cash bonus,
which will be treated as a forgivable loan. The bonus will be forgiven over a
thirty-six month, straight-line schedule. In the event you voluntarily terminate
your employment or your employment is terminated for performance prior to the
thirty-six months from the date of the loan, the unforgiven portion of the loan
would be payable to Biogen within six months of your termination date.

Upon employment you will be granted an option to purchase up to 100,000 shares
of the common stock of Biogen, Inc. This option grant is subject to the approval
of the Stock and Option Plan Administration Committee of the Board of Directors,
which we expect to be forthcoming. The exercise price per share will be the
average of the high and low sale prices as reported in the National Association
of Securities Dealers Automated Quotation System ("NASDAQ") on the date you
commence employment. The option will have a ten-year term and vest over a
five-year period at the rate of 20% per year starting on the anniversary of the
date you commence employment.

You will be able to choose from a menu of benefit options through our flexible
benefits program, BioChoice. These include health care, life, dependent life,
and disability insurance as well as two flexible spending accounts, Med-Flex and
D-Flex, for eligible medical or dependent care expenses. At the beginning of the
calendar quarter following your date of hire you will be eligible to participate
in Biogen's pension and 401(k) savings plans. You are also entitled to twenty
vacation days per year accrued on a monthly pro-rated basis. Biogen also offers
a variety of additional benefits including tuition reimbursement, childcare and
elder care referral service, educational matching gifts, credit union, and group
homeowners and automobile insurance. You will receive more detailed information
regarding your benefits on your first day at work.

<PAGE>   2

Mr. Kees Been
Page 2
July 19, 1999

Biogen will provide relocation benefits to facilitate your move from St. Louis,
MO. A relocation counselor will contact you to discuss the relocation program,
and will send you a package detailing the relocation policy to you. Please read
the policy carefully as it will outline the parameters of these benefits and
detail the reimbursement process. These benefits will include the reimbursement
of many of the costs associated with both the sale of your current house and the
purchase of a new house in the Boston area; home-finding; and movement of your
household goods.

As an additional provision of your relocation benefits, Biogen will pay you a
lump sum of $40,000 on employment to be used for your temporary living in Boston
and to also allow you round trip flights to St. Louis pending your family's
eventual relocation to the Boston area. In the event your family relocates to
the Boston area prior to your fully expending the $40,000, any unused amount of
this lump sum payment will be yours to retain with no repayment obligation to
Biogen.

In the event you voluntarily terminate your employment or your employment is
terminated for performance within two years from the date that you start your
new job, you will be required to repay to Biogen some or all of the relocation
expenses that you have incurred.

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 will be eligible for the following:

-    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 bonus,
     if there is one. I can outline the details of the program if you are
     interested in participating.

-    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.

-    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. In the event you choose to
     continue the services provided by your current tax advisor, Biogen will
     reimburse you the cost of these services up to but not exceeding the amount
     incurred by Biogen through PricewaterhouseCoopers.

<PAGE>   3

Mr. Kees Been
Page 3
July 19, 1999

-    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., 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.

Employment with Biogen is contingent on the satisfactory completion of a
pre-employment drug screening test at least five business days prior to your
start date. We require all new employees to sign an Intellectual
Property/Confidentiality Agreement on the first day of employment. You will also
be required to complete a medical history review with our occupational health
department, which is scheduled after your first day of employment.

It is Biogen's policy to comply with federal, state and local guidelines
applicable to its facilities and to take all reasonable steps to ensure the
health and safety of Biogen employees. Consistent with this policy, your normal
duties may require you from time to time to attend meetings or perform functions
in any or all of Biogen's facilities.

The Federal Government requires you to provide proper identification verifying
your eligibility to work in the United States. Please bring the appropriate
identification with you on your first day of employment.

On behalf of Jim Vincent, we very much look forward to your positive response
and to your joining us as early as practical. We are confident that you will
make a significant contribution to Biogen's future success.

<PAGE>   4

Mr. Kees Been
Page 4
July 19, 1999

Please confirm your acceptance by signing this offer letter and noting your
preferred start date. Also, please sign the enclosed drug screen authorization
form and then return both signed documents to me in the enclosed self addressed,
stamped envelope. The other original offer letter is for your records.

                                               Sincerely,

                                               /s/ Frank A. Burke, Jr.

                                               Frank A. Burke, Jr.

FAB:bk
cc:    Mr. James L. Vincent

 I am pleased to accept the offer of employment described above.

ACCEPTED:

/s/ Kees Been
------------------------------------------------------------------------
Kees Been                                           Preferred Start Date

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