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                                                                  EXHIBIT 10.51

[BIOGEN LOGO]                                   Vice President, Human Resources
_______________________________________________________________________________




                                          June 22, 2001




Mr. Hans-Peter Hasler
653 Augusta Court
Berwyn, Pennsylvania 19312

Dear Hans-Peter:

I am pleased to confirm Jim Mullen's offer to you on behalf of Biogen.  You are
offered the position of Executive Vice President, Commercial Operations of
Biogen at an annual salary of $350,000.  You will report directly to Jim Mullen
and you will become a member of the Operating Team.  Pursuant to Biogen's
compensation policy, you will be eligible for a merit salary review at year-end
2001 and annually thereafter.  You will also be eligible to participate in
Biogen's annual incentive compensation plan with a 50% target bonus.  For the
year 2001, your bonus will be guaranteed at $100,000 and will be payable at a
time and manner consistent with other peer executives.  Your principal place of
employment will be at Biogen's headquarters in the Cambridge, Massachusetts
area.

Upon employment, you will receive $500,000 as a one-time special cash payment,
split into two components:

         -  $250,000 as an interest-free forgivable loan (the "Forgivable Loan")
            which forgives over a thirty-six month, straight-line schedule;
         -  $250,000 as an interest-free mortgage loan (the "Mortgage Loan")
            repayable to Biogen the sooner of eighteen months following the
            point of time the after-tax profit of your vested stock options
            exceeds $1,000,000 or five (5) years from your date of employment.

In the event that you voluntarily terminate your employment, or Biogen
terminates your employment for poor performance or cause, prior to the
thirty-six months from the date of the Forgivable Loan, the unforgiven portion
of the Forgivable Loan would be payable to Biogen within six months of your
termination date.

If you voluntarily terminate your employment, or if your employment is
terminated by Biogen for poor performance or cause, prior to the repayment of
the Mortgage Loan, you will be required to repay Biogen the entire amount of the
Mortgage Loan the sooner of six months following the termination of your
employment or the closing on the sale of the property by which the loan is then
secured.

The Stock and Option Plan Administration Committee of the Board of Directors has
approved the grant of an option to you to purchase 150,000 shares of the common
stock of Biogen Inc., effective on the date you commence employment.  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 four-year period at the rate of 25% per year
starting on the anniversary of the date you commence employment.  In the future,
based on your performance, you will be eligible for merit stock option grants
beginning at year-end 2002 pursuant to Biogen's merit stock option policy and
in-line with awards granted to the executive officers below the President & CEO.
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[BIOGEN LOGO]
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     Mr. Hans-Peter Hasler
     June 22, 2001
     Page 2

     The Board of Directors' recently approved an amendment to our stock option
     plan which provides the acceleration of stock option vesting based on the
     change in control of the corporation and a corresponding adverse impact to
     the individual officer which otherwise causes the individual to lose the
     unvested stock option grants made by Biogen.

     As an officer of the Company, you will be required to participate in our
     program for executive stock ownership, which we are currently revising and
     expect to have approved by the end of September. We believe it is very
     important that our key leaders have personal stock ownership.

     You will be able to choose from a menu of benefit options through our
     flexible benefits program, BioChoice. These include health care, 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 employment you
     will be eligible to participate in Biogen's pension plan. Biogen also
     offers a variety of additional benefits including 401(k) savings plans,
     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
     of employment.

     Biogen will provide relocation benefits to facilitate your move from
     Berwyn, PA. 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.

     Additionally, in your new role you will be provided the following:

     -    VACATION: You are entitled, during your employment with Biogen, to
          four (4) weeks vacation, accrued on a monthly pro-rated basis.

     -    SUPPLEMENTAL SAVINGS PLAN: You are entitled, during your employment
          with Biogen, 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, during your employment with
          Biogen, 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, during your employment with
          Biogen, to the preparation and/or review, including review of the
          estimated taxes of your annual Federal and State tax returns, which is
          currently administered through Price WaterhouseCoopers. The cost of
          this service is covered by Biogen. In the event that 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 Price WaterhouseCoopers.



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[BIOGEN LOGO]
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Mr. Hans-Peter Hasler
June 22, 2001
Page 3


- INVOLUNTARY TERMINATION: If your employment with Biogen is terminated by
  Biogen without 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
  position.

Commencement of 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 may 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 Mullen, 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.

Please confirm your acceptance by signing this offer letter and noting your
preferred start date. I also invite you to complete the enclosed invitation to
self identify. Please complete the employment application and the drug screen
authorization forms and then return all signed documents to me in the enclosed
self addressed, stamped envelope. The other original offer letter is for your
records.

                              Sincerely,




                              Frank A. Burke, Jr.
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[BIOGEN LOGO]
________________________________________________________________________________

  Mr. Hans-Peter Hasler
  June 22, 2001
  Page 4




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

  ACCEPTED:

/s/ Hans-Peter Hasler
______________________
  Hans-Peter Hasler