<PAGE>

Brian McNamee
May 2, 2001

                                                                   Exhibit 10.49

May 2, 2001



Brian McNamee



Dear Brian:

I am pleased to offer you the position of Senior Vice President, Human Resources
reporting to me.  You will be based at our Thousand Oaks, California facility.

Base Salary and Annual Incentive
--------------------------------

Your monthly salary will be $29,167.  In addition, you will be entitled to a
$250,000 bonus which will be paid within 30 days of your start date.  As an
Senior Vice President, you will be eligible to participate in Amgen's Management
Incentive Plan (the "MIP") pursuant to the terms of the MIP.  Your annual target
incentive opportunity will be 63% of your base salary earnings during the year.
Your performance against pre-established goals and Amgen's performance will
determine your actual incentive each year.  However, in order to ease your
transition to a new company and a new area, Amgen will guarantee a minimum
incentive payment of $100,000 (payable in March 2002) for 2001 and $200,000
(payable in March 2003) for 2002. You must be actively employed by Amgen on
December 31, 2001 and on December 31, 2002 to receive the guaranteed payments
for 2001 and 2002, respectively.

Annual Retention Bonuses
------------------------

On each of the first five one-year anniversaries of your Start Date, Amgen will
pay you a retention bonus in the amount of $100,000, provided that you are
actively employed by Amgen on each such date.  If you are not so employed on
each such date, no portion of the bonus is considered earned or vested and no
prorated payments will be made.

Stock Options
-------------

Subject to the approval of the Compensation Committee of the Amgen Board of
Directors, you will be granted an option to purchase 100,000 shares of Amgen's
common stock at a price equal to 100% of the fair market value on your start
date. This option shall be an incentive stock option to the extent permitted by
law, with the balance being granted as a non-qualified stock option.  This
option shall be vested at a rate of 25% per year for four years, beginning one
year from the date of grant, and the option will expire seven years from the
date of grant.

In addition, you will also be eligible to receive additional stock options as
part of Amgen's Periodic Stock Option Program (the "PSOP").  Grants under the
PSOP are discretionary and are
<PAGE>

Brian McNamee
May 2, 2001

usually made in July of each year, as approved by Amgen's Compensation
Committee. However, for 2001 and 2002, subject to approval by the Compensation
Committee of Amgen's Board of Directors, Amgen will grant you an option to
purchase 90,000 shares in each of these years at a price equal to 100% of the
fair market value on the grant date. This option shall be an incentive stock
option to the extent permitted by law, with the balance being granted as a non-
qualified stock option. These options will be granted pursuant to, and in
conformity with, the terms of the PSOP. These PSOP options will vest 20% per
year for five years, beginning one year from the date of grant, and the options
will expire seven years from the date of grant. You must be actively employed by
Amgen on the PSOP grant date in each year to receive the PSOP grant for that
year.

Termination
-----------

If, within the first five years of your employment with Amgen, either: (i) Amgen
terminates your employment without Cause, as defined below, or (ii) you resign
your employment due to a reduction of your duties or your base salary or annual
target incentive opportunity under the MIP, then you will be entitled to three
years of base salary and target incentive paid monthly and health care coverage
unless coverage is obtained from another employer, but only if you sign a
                                                       ----
general release form furnished to you by Amgen.  If you intend to resign your
employment for reduction of duties or compensation, you must notify the Company
in writing.  If Amgen fails to cure or remedy your reason for resignation within
thirty (30) days of its receipt of your notification and you still choose to
resign, you must do so within fifteen (15) days of Amgen's failure to cure or
remedy your reason.

As a Senior Vice President at Amgen, you will also be eligible to participate in
the Amgen Inc. Change of Control Plan.  A copy of the Plan and a summary are
enclosed.  If upon termination, you are also entitled to receive severance
benefits under the Amgen Inc. Change of Control Severance Plan (the "COC Plan")
on account of a termination covered by this provision, you will be paid the
greater of the amount provided above or provided in the COC Plan, but not both
amounts.

Solely for the purpose of this provision and Attachment 3, "Cause" means (i)
your conviction of a felony,  (ii) the engaging by you in conduct that
constitutes willful gross neglect or willful gross misconduct in carrying out
your duties to Amgen, resulting, in either case, in material economic harm to
Amgen, unless you believed in good faith that such conduct was in, or not
contrary to, the best interests of Amgen, (iii) your material breach of any of
the terms of this letter agreement or the Proprietary Information and Inventions
Agreement or (iv) your failure to follow any lawful directive of Amgen's Chief
Executive Officer with respect to your employment.  For purposes hereof, no act,
or failure to act, on your part shall be deemed "willful" unless done, or
omitted to be done, by you not in good faith.

Benefits
--------

You will also have the opportunity to participate in our comprehensive benefits
program.  Amgen's excellent health care plan currently includes medical, dental,
and vision coverage for you and your eligible dependents.  Amgen currently pays
the major expense for these programs
<PAGE>

Brian McNamee
May 2, 2001

while staff members share through payroll deductions. Please be advised that in
order for you and your dependents to be eligible for Amgen's medical coverage
you must:

    1.  Report to work at Amgen or another location to which you are required to
        travel and perform the regular duties of your employment.

    2.  Contact the Amgen Benefit Center at Fidelity, 1-877-999-7779, to enroll
        within 31 days of your hire date.

    3.  Meet all other eligibility requirements under the plan.

Amgen's Retirement & Savings 401(K) Plan provides an opportunity for staff
members to save up to 15% of their pay on a tax deferred basis. (Maximum
statutory individual limit for 2001 will be $10,500).  Amgen will also
contribute to your 401(K) account to help you save for your future financial
goals.  The benefits, services and programs are summarized in the enclosed
brochure called "Welcome to Amgen - Total Compensation and Benefits at Amgen."
You will also be eligible for Amgen's Supplemental (401K) Retirement Plan (the
"SRP").

You will also be entitled to senior executive physicals, financial counseling
and tax preparation services at the same level as all other Amgen senior
executives.  When you travel on Amgen business, you may travel first class,
provided that you make arrangements for such travel with Amgen's designated
travel agents and pursuant to Amgen's travel and business expense policies.  You
will be responsible for paying all applicable income and employment taxes on
your Amgen compensation and benefits and will be subject to all income and
employment tax withholding, except as expressly provided otherwise with respect
to certain relocation benefits described in Attachment 2.

Required Documents
------------------

Enclosed and included as part of this offer (Attachment 1) is information
regarding Amgen's Proprietary Information and Inventions Agreement, the
Immigration Reform & Control Act, and a packet of materials entitled
"Arbitration of Disputes - Amgen Inc." which includes a Mutual Agreement to
Arbitrate Claims.  This offer is contingent upon your completing the items
described in Attachment 1.

Relocation
----------

Also enclosed and included, as part of this offer (Attachment 2), is information
about the main points of the relocation assistance which Amgen will provide to
you to relocate to the "local area."  The brochures included describe each
component in more detail.  Upon acceptance of this offer, please fill out the
attached "Moving Forward...With Amgen" acceptance form and fax it to the
Relocation Department at 805/447-1985 to initiate your relocation benefits. Gail
Thomas will contact you as soon as possible to walk you through the process.

Employment At Will
------------------

By signing this letter, you understand and agree that your employment with Amgen
is at-will.  Therefore, your employment can terminate, with or without Cause,
and with or without notice, at
<PAGE>

Brian McNamee
May 2, 2001

any time, at your option or Amgen's option, and Amgen can terminate or change
all other terms and conditions of your employment, with or without Cause, and
with or without notice, at any time. This at-will relationship will remain in
effect throughout your employment with Amgen Inc. or any of its subsidiaries, or
affiliates. This letter constitutes the entire agreement, arrangement and
understanding between you and Amgen on the nature and terms of your employment
with Amgen. This letter supersedes any prior or contemporaneous agreement,
arrangement or understanding on this subject matter. By executing this letter as
provided below, you expressly acknowledge the termination of any such prior
agreement, arrangement or understanding. Also, by your execution of this letter,
you affirm that no one has made any written or verbal statement that contradicts
the provisions of this letter. The at-will nature of your employment, as set
forth in this paragraph, can be modified only by a written agreement signed by
both Amgen's Chief Executive Officer and you which expressly alters it. This at-
will relationship may not be modified by any oral or implied agreement, or by
any Company policies, practices or patterns of conduct.

We are enthusiastic about the contribution you can make, and we believe that
Amgen can provide you with attractive opportunities for personal achievement and
growth.  Please retain the original offer letter for your records.  If you have
any questions regarding this offer, please contact John Hillins at (805) 447 -
7456.


Sincerely,
/s/ Dennis Fenton




/s/ Brian McNamee              5/5/2001
----------------------------------------
Signature of Acceptance         Date

Enclosures
<PAGE>

Brian McNamee
May 2, 2001

                                  ATTACHMENT 2
                                  ------------



RELOCATION ASSISTANCE COVERAGE

All relocation expense coverage to be provided as a part of your Amgen
employment offer is outlined in this attachment.  This relocation expense
coverage is designed to offset most of the cost of your relocation.  However, as
a new staff member, it is expected that you will make every effort to reduce or
eliminate relocation expense wherever possible.

Please Note: Upon acceptance of this offer, please fill out the attached "Moving
Forward...With Amgen" acceptance form and fax it to the Relocation Department at
805/447-1985 to initiate your relocation benefits. Gail Thomas, our relocation
manager, will contact you as soon as possible to walk you through the process.

     Marketing Assistance, Home Sale, and Home Sale Incentive Program
     ----------------------------------------------------------------

     A Marketing Assistance Program is available to assist in the sale of your
     current primary residence.  Also, through the Home Sale Program, we will
     offer you the opportunity for a third party purchase of your current
     primary residence if you are unable to sell your home within 90 days.
     Under this program, an interest-free equity bridge loan is available to
     assist in the purchase of your new residence. The seller's normal, non-
     recurring closing costs associated with the sale of your home (i.e., real
     estate commission, title expense, etc.) will be paid by Amgen through this
     program.  You are also eligible for the Home Sale Incentive Program which
     is designed to reward you for helping expedite the sale of your home.  For
     additional information, and to initiate the program contact  Gail Thomas.
     (805) 447-0397. You must contact her before taking any action to sell
                                          --------------------------------
     your home.
     ---------

     Additionally, should you close escrow on the purchase of a home in the
     Thousand Oaks prior to the sale of your current residence, Amgen will
     reimburse up to 3 months of your current mortgage payment and other
     reasonable related costs (i.e., utilities, prorated taxes, insurance,
     etc.).

     House Hunting Trip
     ------------------

     Amgen will reimburse you for your out of pocket expenses in connection with
     your house hunting efforts. You should call Dollie Grajczak at 805-447-6110
     in Amgen's Corporate Travel Dept. for assistance with your travel plans.
     You will receive reimbursement upon presenting trip receipts (in the form
     of the airline tickets or hotel bill associated with this trip) to our
     Relocation Manager.

     Temporary Living Expenses
     -------------------------

     Temporary living lodging expense will be covered for up to one year in
     Amgen leased lodging units. Since Amgen has contracted for these temporary
     lodging
<PAGE>

Brian McNamee
May 2, 2001

     accommodations, there is no need to make arrangements on your own. The
     Relocation Coordinator will assist in making these lodging arrangements for
     you. Amgen will also determine a per diem allowance, to be paid to you as a
     lump sum for food, telephone and miscellaneous expenses you may incur
     during your temporary living period.

     One-Way Travel Expenses
     -----------------------

     Amgen will reimburse one-way travel expenses for you and your household
     members to take residence in the Thousand Oaks area.  If Amgen has arranged
     for your car to be moved by a moving company, Amgen will also pay for
     rental of one automobile, for up to 14 days You should contact Dollie
     Grajczak at 805-447-6110 in Amgen's Corporate Travel Dept. to make your
     travel reservations.

     Moving Household Goods
     ----------------------

     Amgen will arrange for packing, moving, and unpacking of normal household
     possessions, including up to two automobiles.  Amgen will also pay for up
     to 365 days of storage of household goods, if necessary.  Amgen will
     initiate contact with moving companies and will handle all details with the
     company assigned to your move.

     3-2-1 Mortgage Subsidy Program
     ------------------------------

     To assist you in the purchase of your new home in the Thousand Oaks area,
     Amgen will provide you the option of a temporary mortgage subsidy program.
     To participate in this program, you must obtain your first loan through one
     of the approved lenders with which Amgen has entered into a subsidy
     agreement.

     Through this program, Amgen will subsidize your mortgage as follows: 3
     points the first 12 months, 2 points the second 12 months and 1 point the
     third 12 months.  The payment of the mortgage subsidy will be handled
     between Amgen and the Mortgage Lender.

     Also, you will be reimbursed for Mortgage Loan Origination Fees, in an
     amount not to exceed one percent (1%) of the principal balance.  An
     additional Six Hundred Fifty Dollars ($650.00) is granted for other
     Lender's Fees.  These fees include, but are not limited to, fees for the
     appraisal, credit report, tax service fees, processing fees, flood zone
     determination fees, underwriting fees, warehouse fees, rate lock-in fees,
     broker fees, lender document preparation fees, commitment fees, lender
     courier fees, escrow waiver fees, and loan review fees.  You will also be
     reimbursed for the customary non-recurring buyer's closing costs for Escrow
     and/or Title fees and home inspection.

     You will be required to sign an Employee Subsidy Agreement that will detail
     the terms of the program.

     If you choose not to or are unable to utilize the 3-2-1 Mortgage Subsidy
     Program, you will be eligible for reimbursement of up to three points (3%)
     of the mortgage amount for loan origination or discount expense. You will
     also receive reimbursement of the buyer's
<PAGE>

Brian McNamee
May 2, 2001

     normal, non-recurring closing costs associated with the purchase of a home
     in the "local area".

     Please note that you are not required to obtain your first loan through an
     approved lender unless you intend to participate in the 3-2-1 Mortgage
     Subsidy Program

     For more information or to begin the process, please call your Relocation
     Specialist.

     Adjustable Rate Secured Loan
     ----------------------------

     To aid in the purchase of a home in the Thousand Oaks area, Amgen is
     prepared to offer you a five year, adjustable rate loan for up to
     $1,000,000 which will be secured by a second mortgage on your new home.
     However, you will be expected to provide a minimum down payment investment
     of at least 5% of the purchase price from your own funds or other sources
     which are not secured by this home.

     The loan will be funded prior to close of escrow at a date to be determined
     solely by Amgen. This loan will not be funded prior to you beginning your
     employment at Amgen.

     The 2001 rate on the loan is 5.0%.  The rate is adjusted January 1st of
     each year based on the average "Introduction Rates" on adjustable loans as
     offered by California banks and savings & loans.  The most the rate will
     change each year is 1% with a cap of 3% over the life of the initial loan.
     You will be required to make semi-monthly interest only payments by payroll
     deduction.

     Principal and accrued interest on the loan will be due and payable upon the
     earlier of the 5th anniversary of the loan date or 90 days after your
     employment with Amgen terminates for any reason.  You may prepay interest
     or principal on the loan at any time.

     Tax Gross-up Assistance
     -----------------------

     Amgen will provide for tax assistance (gross-up) for the non-deductible
     portion of those reimbursed relocation expenses described in this
     Attachment 2 and which are considered as ordinary income for state or
     federal income tax purposes.

     Duration of Relocation
     ----------------------

     This relocation expense coverage is intended to assist you in getting
     established in your new residence in the Thousand Oaks area as quickly as
     possible.  Therefore, it is required that all relocation assistance
     provided for in this attachment and all expense reimbursements for this
     assistance be completed within one year from your date of hire in your new
     location.