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                                              [BGR I PROPERTY BUILDING NQ LEASE]


                         THIRD AMENDMENT TO TRIPLE NET LEASE

         THIS THIRD AMENDMENT TO TRIPLE NET LEASE (this "Third Amendment") is
entered as of the 31st day of January, 1997, by and between BGR ASSOCIATES, A
CALIFORNIA LIMITED PARTNERSHIP ("Lessor"), and CHIRON CORPORATION, a Delaware
corporation ("Lessee").

         THIS THIRD AMENDMENT IS ENTERED on the basis of the following facts,
intentions and understandings of the parties:

    A.   Lessor is the owner of the improved real property (the "BGR I
Property") in the City of Emeryville, State of California, commonly known as
4560 Horton Street.  The BGR I Property includes the buildings known as
"Building N," "Building Q," "Building R," and "Building E."

    B.   Lessor and Chiron Corporation, a California corporation (predecessor
to Lessee), entered a Triple Net Lease (the "Original Lease") dated May 26,
1989.  Pursuant to the Original Lease, Lessor leased to Lessee (as successor to
Chiron Corporation, a California corporation) all of Building E (referred to as
"Building P" in the Original Lease) and portions of Building N and Building Q.

    C.   Lessor and Chiron Corporation, a California corporation (predecessor
to Lessee), entered a First Amendment to Lease (the "First Amendment") dated as
of October 1, 1993.  Lessor and Lessee also entered a Second Amendment to Lease
(the "Second Amendment") dated as of May 9, 1996.  The Original Lease as
amended by the First Amendment and the Second Amendment is hereinafter referred
to as the "Lease."  Terms which are capitalized in this Third Amendment and not
defined herein shall have the meanings set forth in the Lease.

    D.   Lessor and Cetus Corporation (predecessor to Chiron) entered a lease
(the "Original Building BGR I Property Building R Lease") dated December 17,
1984, pursuant to which Lessor leased to Lessee (as successor to Cetus
Corporation) all of Building R and portions of Building N and Building Q.  The
Original Building BGR I Property Building R Lease, as amended, is referred to in
this Third Amendment as the "BGR I Property Building R Lease."

    E.   Lessor (together with other entities) and Lessee entered an Option
Agreement dated as of March 15, 1995.  Pursuant to the Option Agreement, Lessee
has the option ("Lessee's Purchase Option"), subject to the terms of the Option
Agreement,


                                          1.

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to purchase the Premises (together with other properties leased by Lessor and
affiliates of Lessor).

    F.   As of the date of this Third Amendment, (i) Lessee is leasing all of
Building N and Building Q pursuant to the Lease and the BGR I Property Building
R Lease and (ii) has contracted to lease all of Building N and Building Q for
the remainder of the term of the Lease (I.E., the Term).

    G.   Lessor and Lessee desire to amend the Lease to provide that, so long
as Lessee's Purchase Option remains in effect, Lessee will not be required to
obtain Lessor's consent for non-structural repairs or alterations to the
Premises which do not affect any central or core building systems and which do
not affect the exterior architectural appearance of the Premises.

    NOW, THEREFORE, IN CONSIDERATION of mutual covenants and promises the
parties, the parties agree as follows:

         1.   ALTERATIONS.  So long as Lessee's Purchase Option is in full
force and effect, Section 7.3 of the Original Lease is amended to provide as
follows:

              a.   LESSOR'S CONSENT.   Lessee shall not be required to obtain
Lessor's consent with respect to any change, alteration or addition to the
Premises which (i) is non-structural, (ii) does not affect any central or core
building systems, and (iii) does not affect the exterior architectural
appearance of the Premises.

              b.   NO COMPLETION BOND.  Lessee shall not be required to provide
a lien and completion bond to Lessor in connection with any change, alteration
or addition.

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



                                          2.

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         3.   REMAINDER UNAFFECTED.  Except as provided in this Third
Amendment, the Lease shall remain in full force and effect and unamended.

         IN WITNESS WHEREOF, the parties hereto have executed this Third
Amendment, on the date(s) set forth below, as of the day and year first above
written.

                        BGR ASSOCIATES, A CALIFORNIA LIMITED PARTNERSHIP

                             /s/ Richard K. Robbins
                        -------------------------------------------------
                        By:  Richard K. Robbins
                             Its:  Managing General Partner
                        Date:  1/28/97
                             ---------------------------------------------


                        CHIRON CORPORATION, a Delaware corporation



                        By:  /s/ Dennis L. Winger
                             ---------------------------------------------
                             Its: DENNIS L. WINGER, Senior VP Finance & Admin
                                  ----------------------------------------
                        Date:  1/30/97
                             ---------------------------------------------

                                          3.