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                                                                   Exhibit 10.52

                                               Vice President, Human Resources





                                October 19, 2001


Hans-Peter Hasler
25 Lovers Lane
Southborough, MA 01772

Dear Hans-Peter:

As an Executive Vice President of Biogen, we know you will continue to be a key
contributor to Biogen's mission, vision, and values. On August 1, 2001, you
joined Biogen at the Executive Vice President level. Since then, we have been in
the process of conducting a comprehensive review of the compensation and
benefits associated with various executive levels at our company.

As a result of our review, I am pleased to note you are eligible for the
following compensation/and or benefits as outlined below.

TARGET BONUS: Your incentive target is 50% of your base salary.

LONG TERM INCENTIVES: Biogen has adopted a new approach to communicating and
granting stock options to employees. You will be eligible for an annual merit
stock option grant, based on a design which emphasizes the future expected
value of the grant. The details of how we will communicate and value options
will be outlined in December.

CHANGE OF CONTROL: You have been designated as a "Designated Employee" for
purposes of Biogen's 1985 Non-Qualified Stock Option Plan. If at any time
within two years following a Corporate Transaction (as defined in the stock
option plan) your employment with Biogen is terminated by Biogen other than for
cause, then each outstanding option held by you will automatically accelerate
so that the option immediately becomes fully exercisable and may be exercised
for a period of one year following the termination of your employment or, if
earlier, until the expiration of the option. Please read the stock option plan
for more details about the rights of a Designated Employee in the event of a
Corporate Transaction and any applicable limitations.

VACATION: You are entitled to an additional week of vacation each year. This
week is over and above the company's normal vacation schedule (which is based
upon years of service).

SUPPLEMENTAL SAVINGS PLAN: You will be entitled to participate in the Voluntary
Executive Supplemental Savings Plan. This plan allows participants to defer
[pretax] base salary and future bonuses. Enrollment in this plan takes place in
December each year. You will be provided additional information on the plan at
that time.

LIFE INSURANCE: You will have life insurance coverage that is the greater of
$1,000,000 or three times your annual salary and subject to the normal medical
standards of the life insurance policy.
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                                                Vice President, Human Resources





SEVERANCE:  You will have a Minimum severance benefit that is the greater of the
severance outlined in your letter dated June 22, 2001 or the severance outlined
in the Executive Severance document, which has been included with this letter.

EXECUTIVE STOCK OWNERSHIP:  You will be expected to acquire and maintain a
personal financial interest in Biogen.  Ownership is defined as stock held
outright, stock held in the 401(k) plan and stock held under the Employee Stock
Purchase Plan.  Vested but unexercised shares would also be counted toward
ownership guidelines.  You will be required to maintain a financial interest of
six times base salary.  You will have a five-year period from the date of your
appointment to Vice President to obtain this ownership level.

TAX PREPARATION:  You are eligible to have your federal and state income taxes
prepared by PricewaterhouseCoopers.

On behalf of Jim Mullen, please accept my sincere congratulations and best
wishes for future success with Biogen.



                                              Sincerely,



                                              /s/ Frank A. Burke, Jr.
                                              Frank A. Burke, Jr.