FIRST AMENDMENT TO TRIPLE NET LEASE



         THIS FIRST AMENDMENT TO TRIPLE NET LEASE ("First Amendment") is
entered into as of this 10th day of September, 1996, by and between BGR
Associates III, a California limited partnership ("Lessor"), and Chiron
Corporation, a Delaware corporation, as successor to Cetus Corporation
("Lessee").  Lessor and Lessee are collectively referred to herein as the
"Parties".

         THE PARTIES ENTER INTO THIS FIRST AMENDMENT on the basis of the
following facts, intentions and understandings:

         A.   The Parties entered into a Triple Net Lease ("Lease") dated
January 20, 1989 covering certain real property more particularly described in
EXHIBIT A to the Lease (the "Premises").

         B.   The Parties have requested, and the City of Emeryville has
approved, a Certificate of Compliance and Lot Line Adjustment ("Lot Line
Adjustment") which relocates a portion of the boundary line between one of the
parcels comprising the Premises (Parcel 5 as described in EXHIBIT A to the
Lease) and an adjacent parcel outside the Premises (which is owned by Lessee)
with the result that the Premises will be reduced in size by approximately four
thousand three hundred eighty-five (4,385) square feet.

         C.   The Parties desire to enter into this First Amendment in order to
modify the description of the Premises to take into account the effect of the
Lot Line Adjustment.

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

         1.   The legal description of Parcel Five in EXHIBIT A to the Lease is
hereby replaced in its entirety with the following legal description:

    PARCEL FIVE:

    PARCEL A, AS SAID PARCEL IS SHOWN UPON PARCEL MAP NO. 2108, FILED ON JUNE
    17, 1977 IN BOOK 97 OF PARCEL MAPS AT PAGES 40 - 41, IN THE ALAMEDA COUNTY
    RECORDER'S OFFICE.

    EXCEPTING THEREFROM THE FOLLOWING DESCRIBED PARCEL:



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    ALL OF SAID PARCEL A, LYING EASTERLY OF THE NORTHERLY PROLONGATION OF THAT
    CERTAIN COURSE "SOUTH 17 DEG.  32' EAST 582.48 FEET", AS SAID COURSE IS
    SHOWN UPON SAID PARCEL MAP.

         2.   EFFECT OF FIRST AMENDMENT.  Except as specifically provided in
this First Amendment, the Lease shall remain in full force and effect and
unamended.

         3.   COUNTERPARTS.  This First Amendment may be executed in one or
more counterparts, each of which shall be deemed an original and all of which
together shall constitute one and the same instrument.

         IN WITNESS WHERE0F, the Parties have executed this First Amendment to
be effective as of the day and year first above written.

                             LESSOR

                             BGR ASSOCIATES III, a California limited
                             partnership



                             By:  /s/ Richard K. Robbins
                                --------------------------------------

                             Its: Managing General Partner
                                 -------------------------------------

                             LESSEE

                             CHIRON CORPORATION, a Delaware corporation



                             By: /s/ Dennis L. Winger
                                --------------------------------------

                             Its: CFO, SVP, Finance & Admin
                                 -------------------------------------

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