<PAGE>

                                                              EXHIBIT 10.3

                                 [LETTERHEAD]

August 5, 1999

Mr. Kevin Starr
Chief Financial Officer
Millennium Pharmaceuticals, Inc.
75 Sidney Street
Cambridge, MA  02139

Dear Kevin:

We are pleased to advise you that our Senior Management Committee has approved a
firm commitment for a lease line to Millennium Pharmaceuticals, Inc., (the
"Lessee"), to be funded by us or our nominee, on the following terms and
conditions.

1.       TRANSACTION: The transaction is structured as a true lease in which the
         Lessor will be entitled to claim and retain all of the tax benefits
         associated with ownership of the equipment. The lease will be a net
         lease in which the Lessee will be responsible for all the expenses
         relating to the equipment and the transaction; including equipment
         maintenance, insurance coverage, payment of personal property taxes,
         recording and appraisal fees and other expenses. Information provided
         to GE Capital may be used by GE Capital or it potential nominees in
         evaluating the transaction.

2.       LESSEE:  Millennium Pharmaceuticals, Inc.

2a.      LESSOR: GE Capital or its nominee.

3.       ORIGINATOR:  Meier Mitchell & Company

4.       EQUIPMENT: Various new laboratory, test, office and other non-computer
         equipment, all of which must be acceptable to Lessor. Transportation
         and other soft costs cannot exceed ten percent (10%) of the Acquisition
         Cost.

5.       DELIVERY: All Equipment must be delivered, accepted and scheduled on or
         before June 30, 2000.

6.       ACQUISITION COST:  $15,000,000 (Lessor will use its best efforts to
         syndicate an additional $5 million)

7.       TERM:  Five (5) years from the Base Lease Commencement Date.

8.       BASE LEASE COMMENCEMENT DATE: The first day of the month following
         scheduling.

<PAGE>

9.       BASE LEASE RENTAL PAYMENT: Lessee will be required to make sixty (60)
         monthly rental payments, each payable in advance, and equal to the
         following percentage of the Acquisition Cost depending upon the Date of
         Scheduling.

         DATE OF SCHEDULING                            RENTAL FACTOR
         Q3 1999                              30 @ 1.6680%, then 30 @ 2.0386%
         Q4 1999                              30 @ 1.6602%, then 30 @ 2.0291%
         Q1 2000                              30 @ 1.6862%, then 30 @ 2.0610%
         Q2 2000                              30 @ 1.6768%, then 30 @ 2.0494%

         The Base Lease Rental Payment was calculated using current money market
         rates; however, money market conditions at the Date of Scheduling will
         control the final Base Lease Rental Payment that is fixed for the Term.
         See "Adjustments to the Base Lease Rental Payment" below.

10.      INTERIM RENTAL PAYMENT: From the Date of Scheduling to the Base Lease
         Commencement Date, the Lessee will be required to make Interim Rental
         Payments at an interest rate equal to the commercial paper rate at the
         Date of Scheduling plus 225 basis points.

11.      TAX BENEFITS: Lessee will represent and warrant that the depreciation
         deductions arising our of the ownership of all of the Equipment will be
         for the account of Lessor and will be recognized over a five (5) year
         period on a 200% declining balance switching to straight-line (DDB/SL)
         formula using the half-year convention. A Federal corporate tax rate of
         35% for 1999 and thereafter was assumed in calculating the Base Lease
         Rental Payment. Lessee will indemnify Lessor for any loss of any Tax
         Benefits caused by an act, omission or misrepresentation of Lessee.

12.      ADMINISTRATIVE FEE: An Administrative Fee equal to $100,000 has been
         paid to GE Capital. Of that amount $75,000 is considered earned but
         will be applied pro rata to the first monthly rental payment under each
         schedule. The remaining $25,000 will be treated in a similar fashion
         upon GE Capital's successful syndication of the additional $5 million.

13.      LESSEE OPTIONS AT LEASE EXPIRATION: At the expiration of Term, Lessee
         will have the following options:

         A.       Renew all the  Equipment  under the lease for ten (10)  months
                  at 1.2% of  Acquisition  Cost per month,  in  advance,  after
                  which time the Lessee may:

                  1.       Return all the Equipment to the Lessor;

                  2.       Renew all the Equipment under lease at a term and
                           rate to be negotiated by the parties based upon the
                           then remaining useful life and fair market value of
                           the Equipment; or

                  3.       Purchase all the Equipment at its then fair market
                           value; OR

         B.       Purchase all the  Equipment  for the greater of ten percent
                  (10%) of original  Acquisition  Cost or its then fair market
                  value.  The Lessee may select the qualified appraiser.

14.      ADJUSTMENTS TO THE BASE LEASE RENTAL PAYMENT: The Base Lease Rental
         Payment stated above reflects current money market rates as indicated
         by the yield to maturity of 5.36% for three-year Treasury Notes (the
         Reference Yield).

<PAGE>

The Table below sets forth the U.S. Treasury Notes with similar then remaining
lives to maturity which will be used to establish the final Base Lease Rental
Payment, depending upon the date of scheduling

         DATE OF SCHEDULING                     APPLICABLE TREASURY NOTE

                                             COUPON      MATURITY

         July 1 to September 30, 1999         6.25%     August 2002
         October 1 to December 31, 1999       5.75%     November 2002
         January 1 to March 31, 2000          6.25%     February 2003
         April 1 to June 30, 2000             5.50%     May 2003

The Base Lease Rental Payment actually used will be that stated above, increased
or decreased .00198% for the first 30 payments and .00242% for the last 30
payments for each 5 basis point change in the yield to maturity of the
Applicable Treasury Note from the Reference Yield. The yield to maturity of the
Applicable Treasury Note used to calculate the adjustment to the Base Lease
Rental Payment will be the yield quoted in the most recently published issue of
THE WALL STREET JOURNAL on the Date of Scheduling.

15.      ELECTRONIC PAYMENT SYSTEM: All payments to GE Capital shall be made
         under an electronic payment system at no cost to the Lessee.


16.      FINANCIAL COVENANTS: Lessee agrees that its (a) unrestricted cash,
         cash equivalents and marketable securities shall always total at least
         $30,000,000; (b) tangible net worth shall always be at least
         $30,000,000; (c) current ratio shall always be at least 2.0:1 and (d)
         total liabilities/tangible net worth ratio shall never exceed 1.0:1.
         Unrestricted cash and cash equivalents and marketable securities will
         be defined as being net of any non-GE Capital contingent liability
         associated with other cash triggers or pledge agreements. If for any
         reason Lessee does not meet these covenants, Lessee will provide
         Lessor with an irrevocable letter of credit in a form acceptable to
         Lessor and from a bank acceptable to Lessor equal to Lessor's then
         stipulated loss value on all schedules. Lessee's Chief Financial
         Officer or other senior officer will provide Lessor with a monthly
         compliance statement in a form acceptable to Lessor. The requirement
         for these covenants shall be dropped upon the expiry of all schedules
         previously funded under the lease agreement between Lessor and Lessee
         if no event of default exists at that time and Lessee has paid as
         agreed.

17.      YEAR 2000 COMPLIANCE: ALL OF THE TERMS AND CONDITIONS OF THIS PROPOSAL
         HAVE BEEN MADE ON THE ASSUMPTION THAT THE ASSETS RELATED HERETO ARE
         YEAR 2000 COMPLIANT. THE TERM "YEAR 2000 COMPLIANT" SHALL MEAN THAT THE
         ASSETS HAVE BEEN DESIGNED AND/OR ENGINEERED SUCH THAT THEY WILL, ON OR
         AFTER THE DATE DECEMBER 31, 1999, CONTINUE TO FULLY FUNCTION IN THE
         MATTER INTENDED, AND PERFORM ALL OF THE DUTIES THAT THEY ARE INTENDED
         TO FULFILL, BY THE USER THEREOF ON THE COMMENCEMENT OF THE TRANSACTION
         CONTEMPLATED BY THIS LETTER.

18.      ECONOMIC OBSOLESCENCE: Anytime after the first anniversary of the Base
         Lease Commencement Date of any schedule funded hereunder, Lessee may
         terminate the lease with respect to any Equipment which has become
         obsolete or surplus to Lessee's needs. Lessee will be responsible for
         selling this equipment and shall pay Lessor the attendant termination
         value for that Equipment.

This firm commitment has been rendered in express reliance on the financial or
other statements respecting the conditions, operation, and affairs of the
Lessee, or respecting the equipment to be leased, which Lessee has previously
provided to us, and is based on the understanding that Lessee has committed

<PAGE>

to complete the transaction with us or our nominee. The Lessor's commitment
is subject to the condition that there shall be no material adverse change in
either (i) the business or financial condition of the Lessee or (ii) proposed
Federal tax law, prior to any funding under the loan.

An express condition of the commitment is that all documentation be satisfactory
to our counsel.

This firm commitment will expire on August 13, 1999 unless you acknowledge your
receipt hereof and acceptance by executing the enclosed copy of this letter and
returning it to us by that date. If accepted by you, this commitment will expire
on June 30, 2000.

                                                      GE Capital

                                                      By: /s/ JAMES F. SIEGAL
                                                         -----------------------
                                                      Richard L. Dauphinais
                                                      Senior Transaction Manager
                                                      Third Party Originations

ACCEPTED BY THE LESSEE:

By: /s/ MICHAEL FALVEY
Date: 8/9/99