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                                                                   EXHIBIT 10.43

October 19, 2001


Peter Kellogg
14 Scotch Pine Circle
Wellesley, MA  02481

Dear Peter:

I am pleased to confirm Jim Mullen's conversation appointing you to Executive
Vice President & CFO. This appointment recognizes the increasing importance of
your role and your leadership capabilities. As an Executive Vice President of
Biogen, we know you will continue to be a key contributor to Biogen's mission,
vision, and values.

I am also 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. SEVERANCE: You will have a minimum
severance benefit that is the greater of the severance outlined in your letter
dated June 21, 2000 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


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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.

/bk
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