Document:

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                            [BIOGEN, INC. LETTERHEAD]

                                                                   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 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"), provided, that, notwithstanding the extension term specified in the
Letter Agreement, your chairmanship will continue after the end of the Initial
Chairmanship Term on February 14, 2001 until the Annual Meeting to be held in
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.

Date: March 10, 2000                          By:  /s/ Roger H. Morley
     ------------------                          -------------------------------
                                                  Roger H. Morley
                                                  Chairman, Compensation and
                                                  Management Resources Committee

Date: March 10, 2000                          By:  /s/ Harold W. Buirkle
     ------------------                          ---------------------------
                                                  Harold W. Buirkle
                                                  Member, Compensation and
                                                  Management Resources Committee

<PAGE>   2

Date: March 10, 2000                          By:  /s/ Phillip A. Sharp
     ------------------                          -------------------------------
                                                  Phillip A. Sharp
                                                  Member, Compensation and
                                                  Management Resources Committee

Date: March 10, 2000                          By:  /s/ Mary L. Good
     ------------------                          -------------------------------
                                                  Mary L. Good
                                                  Member, Compensation and
                                                  Management Resources Committee

Acknowledged and agreed

/s/ James L. Vincent
--------------------------
James L. Vincent

Date:  March 10, 2000
     ---------------------<PAGE>   1
                            [BIOGEN, INC. LETTERHEAD]

                                                                   EXHIBIT 10.51

March 18, 1993

Mr. James C. Mullen
Vice President, Operations
Biogen, Inc.
14 Cambridge Center
Cambridge, MA 02142

Dear Jim:

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, Operations and an executive officer of Biogen:

1.   LIFE INSURANCE: You will be provided with Biogen's Executive Life Insurance
     coverage of $1,000,000, which combines the existing group term insurance
     provisions with the provisions of an individual term insurance policy.

2.   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.

3.   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 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<PAGE>   1
                            [BIOGEN, INC. LETTERHEAD]

                                                                   EXHIBIT 10.52

                                                       PERSONAL AND CONFIDENTIAL

                                           January 7, 1999

Mr. James C. Mullen
Biogen, Inc.
14 Cambridge Center
Cambridge, MA 02142

Dear Jim:

This confirms your discussion with Jim Vincent related to your new role in the
organization. Effective January 1, 1999, your new title is President and Chief
Operating Officer with a base salary of $400,000 and a target bonus of 60%.
Additionally, you have been granted 175,000 stock options as of January 4, 1999
with a grant value of $14,525,000 (175,000 shares at $83.00).

                                           Sincerely,

                                           /s/ Frank A. Burke, Jr.

                                           Frank A. Burke, Jr.
                                           Vice President, Human Resources
cc:    Personnel File<PAGE>   1

                            [BIOGEN, INC. LETTERHEAD]

                                                                   EXHIBIT 10.53

                                                       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.,

<PAGE>   2

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<PAGE>   1

                            [BIOGEN, INC. LETTERHEAD]

                                                                   EXHIBIT 10.54

                                                       PERSONAL AND CONFIDENTIAL

                                               November 14, 1996

Mr. M. W. Leuchtenberger
Vice President, Marketing and Sales
Biogen Inc.
14 Cambridge Center
Cambridge, MA 02142

Dear Mark:

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, Marketing and Sales and an executive officer of Biogen.

1.   BASE SALARY: Your base salary will be $200,000 (through 1997), and will be
     reviewed at year-end 1997.

2.   PERFORMANCE BONUS: Your performance bonus target is 30% (effective January
     1, 1997).

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

4.   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.

5.   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.

6.   INVOLUNTARY TERMINATION: If you are involuntarily terminated from
     employment with Biogen (other than for cause), Biogen will protect you by
     paying you a

<PAGE>   2

November 14, 1996
Mr. M. W. Leuchtenberger
Page 2

     supplementary amount (the "Supplementary Amount") equal to your then
     present BASE salary for a 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. In the event you have not obtained alternative medical and dental
     insurance coverage at the earlier of twelve months from your termination or
     the commencement of your new job, you shall have the right to continue your
     medical and dental coverages at your sole expense to the extent required by
     the provisions of the Consolidated Omnibus Budget Reconciliation Act of
     1985 ("COBRA"). The COBRA period shall be deemed to have commenced on the
     date of your involuntary termination from employment with Biogen.

     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

/bk

cc:    Personnel File

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