REDACTED

                                                                  EXHIBIT 10.313



                      **CONFIDENTIAL TREATMENT REQUESTED**

                                    AMENDMENT

         This agreement is entered into by and between CHIRON CORPORATION, a
Delaware corporation ("Chiron") and GEN-PROBE INCORPORATED, a Delaware
corporation ("Gen-Probe").

                                    RECITALS

WHEREAS, Chiron and Gen-Probe signed an agreement on June 11, 1998 with
respect to Products for the Blood Screening Field and [PROVISION ASSIGNED]
(the "Agreement").

         WHEREAS, the parties desire to amend the Agreement as set forth in this
Amendment in order to facilitate the purposes of the Agreement.

                                    AGREEMENT

         In consideration of the facts recited above and the mutual commitments
set forth below, the parties agree as follows:

         1.       Article 14 of the Agreement shall be and is hereby amended to
read as follows:

                  "In consideration for the rights originally granted to Chiron
         by this Agreement with respect to TMA and HPA, Chiron shall pay to
         Gen-Probe, [PROVISION ASSIGNED], the following nonrefundable amounts
         upon the occurrence of each event set forth below:

                  (1) [CONFIDENTIAL TREATMENT REQUESTED]; and

                  (2) [CONFIDENTIAL TREATMENT REQUESTED]:

                         (a) (i) [CONFIDENTIAL TREATMENT REQUESTED]

                         (b) [CONFIDENTIAL TREATMENT REQUESTED].

                  (3) [CONFIDENTIAL TREATMENT REQUESTED].

                  (4) [CONFIDENTIAL TREATMENT REQUESTED].







         2.       Except as expressly amended above, the Agreement shall remain
in full force and effect.

                  By the signatures of their authorized officers below,
Gen-Probe and Chiron adopt this amendment as of December 7, 1999.


CHIRON CORPORATION                    GEN-PROBE INCORPORATED


By       /s/ RAJEN DALAL              By       /s/ HENRY L. NORDHOFF
         ----------------                      ----------------------
                                               Henry L. Nordhoff
Title: President, Blood Testing                President and
         Chiron                                Chief Executive Officer




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