<PAGE>

                                                                Exhibit 10.04

                           FIRST AMENDMENT TO LEASE


     THIS FIRST AMENDMENT TO LEASE ("First Amendment") is entered into 
effective as of March 15, 1995, by and between BGR ASSOCIATES II, A 
CALIFORNIA LIMITED PARTNERSHIP ("Lessor"), and CHIRON CORPORATION, a Delaware 
corporation ("Lessee"), with reference to the following facts:

     A.  Lessor and Lessee are parties to that certain Triple Net Lease dated 
May 26, 1989, entered into by Lessor and Lessee's predecessor, Chiron 
Corporation, a California corporation (the "Lease"). The property covered by 
the Lease is located in Emeryville, California, and is more particularly 
described in the Lease. The Term of the Lease expires August 14, 1999; and 
Lessee has the right to extend the Term of the Lease for two (2) terms of 
five (5) years each.

     B.  Lessor, Lessee and certain other parties related to Lessor have 
entered into that certain Option Agreement of even date herewith (the "Option 
Agreement"), pursuant to which Lessor has been granted to Lessee the option, 
subject to the terms and conditions of the Option Agreement, to purchase the 
property covered by the Lease as well as other property in the vicinity 
thereof. As provided for in Section 5.b of the Option Agreement, Lessee has 
agreed to extend the Term of the Lease through August 14, 2000, as part of 
the consideration to Lessor for granting such option to Lessee.

     C.  Lessor and Lessee are entering into this First Amendment pursuant to 
Section 5.d of the Option Agreement to further evidence the extension of the 
Term of the Lease as provided for in the Option Agreement.

     NOW, THEREFORE, in consideration of the foregoing, and for other good 
and valuable consideration, the receipt and adequacy of which are hereby 
acknowledged, Lessor and Lessee hereby agree as follows (capitalized terms 
used herein but not herein defined shall have the meaning ascribed to them in 
the Lease):

     1.  AMENDMENT OF PARAGRAPH 1.2.  Lessor and Lessee hereby agree that, as 
provided in the Option Agreement, the Term of the Lease is extended through 
August 14, 2000; and in furtherance of the foregoing, Lessor and Lessee 
hereby amend Paragraph 1.2 of the Lease by deleting therefrom the words "Ten 
(10) years" at the beginning of said paragraph and substituting therefor the 
words "Eleven (11) years."



<PAGE>

     2.  TERMS OF LEASE.  The terms of the Lease for the period from August 
15, 1999, through August 14, 2000, shall be the same as the terms of the 
Lease prior to August 15, 1999, without any adjustment to the base monthly 
rent for the period from April 15, 1999, through August 14, 2000 (i.e., the 
base monthly rent in effect as of August 15, 1999, shall remain in effect 
through August 14, 2000). In addition, the terms of Paragraph 18 of the Lease 
providing for two (2) extension options of five (5) years each shall remain 
in effect, with the first extension term, if exercised, commencing on August 
15, 2000.

     3.  STATUS OF LEASE.  Except as amended hereby, the Lease remains 
unamended; and as amended hereby, the Lease and all the terms and conditions 
thereof remain in full force and effect.

     4.  COUNTERPARTS.  This First Amendment may be executed in multiple 
counterparts, each of which shall constitute an original hereof, and all of 
which taken together shall constitute one and the same agreement.

     IN WITNESS WHEREOF, Lessor and Lessee have executed this First Amendment 
as of the date first set forth above.

LESSEE:                                       LESSOR:

CHIRON CORPORATION,                           BGR ASSOCIATES II, A CALIFORNIA
a Delaware corporation                        LIMITED PARTNERSHIP

By:  __________________________________       By: ______________________________
                                                  Richard K. Robbins
Its: __________________________________           Managing General Partner


                                         2