Document:

<PAGE>

                                                                        REDACTED

                                                                  EXHIBIT 10.312

                      **CONFIDENTIAL TREATMENT REQUESTED**

                     ADDENDUM TO AGREEMENT OF JUNE 11, 1998

         This agreement is an addendum to the Agreement dated as of June 11,
1998 between GEN-PROBE INCORPORATED, a Delaware corporation, and CHIRON
CORPORATION, a Delaware corporation ("Chiron").

                                    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].

         WHEREAS, Chiron and Gen-Probe attached a brief agreement in lieu of
Schedules B and C (the "Agreement In Lieu of Schedules") to their June 11, 1998
Agreement.

         WHEREAS, Chiron and Gen-Probe now desire to complete the June 11, 1998
Agreement by approving Schedules B and C thereto and to amend the language of
Article 9 of the Agreement in certain respects.

         NOW, THEREFORE, in consideration of the foregoing premises and the
mutual covenants set forth below, the parties hereby agree as follows:

         1.       The first sentence of Section 9.1.6 of the June 11, 1998
Agreement is amended to read as follows:

                  A Disclosing Party shall have the exclusive option to acquire,
                  on Commercially Reasonable Terms, all right, title and
                  interest in any Derivative Invention which represents an
                  improvement to, and as to which any substantial use would
                  infringe a Valid Claim for, that Core Technology of the
                  Disclosing Party which was the subject of the Confidential
                  Information through or with which the Derivative Invention was
                  made and without which it would not have been made.

         2.       Section 9.1.1(b) of the June 11, 1998 Agreement is amended to
add, at the end thereof, the following provision:

                           (iii) Such use or application of a Joint Invention
                  resulting from application of Section 9.1.6 (d) or (e) is made
                  in connection with any product which competes directly with a
                  nucleic acid probe-based assay product of the other party
                  which has been sold as of the Effective Date of this
                  Agreement.

         3.       Section 9.1.1 of the June 11, 1998 Agreement is amended to
add, at the end thereof, the following provisions:

                                       1
<PAGE>

                           (c) Notwithstanding any other provision of this
                  Agreement, the parties agree that a Derivative Invention (as
                  defined in section 9.1.4, below) as to which any substantial
                  use would infringe a claim of [CONFIDENTIAL TREATMENT
                  REQUESTED], shall be owned jointly by the parties and
                  considered a Joint Invention. Gen-Probe waives any and all
                  rights in any such Derivative Invention to the extent (and
                  only to the extent) that such Derivative Invention is used in
                  connection with immunoassays or protein binding assays and
                  such use would infringe [CONFIDENTIAL TREATMENT REQUESTED].
                  Gen-Probe agrees that Chiron shall have exclusive rights with
                  respect to such uses. Chiron waives any and all rights in any
                  such Derivative Invention to the extent (and only to the
                  extent) that such Derivative Invention is used in connection
                  with nucleic acid hybridization assays and such use would
                  infringe [CONFIDENTIAL TREATMENT REQUESTED]. Chiron agrees
                  that Gen-Probe shall have exclusive rights with respect to
                  such uses.

                           (d) Notwithstanding any other provision of this
                  Agreement, the parties agree that a Derivative Invention (as
                  defined in section 9.1.4, below) made by Chiron as to which
                  any substantial use would infringe a Valid Claim of
                  [CONFIDENTIAL TREATMENT REQUESTED], shall be owned jointly by
                  the parties and considered a Joint Invention. Gen-Probe waives
                  any and all rights in any such Derivative Invention to the
                  extent (and only to the extent) that any such Derivative
                  Invention made by Chiron is used in connection with
                  immunoassays or protein binding assays, provided however that
                  nothing contained herein shall constitute a license to Chiron
                  of any rights under [CONFIDENTIAL TREATMENT REQUESTED]. Chiron
                  waives any and all rights in any such Derivative Invention to
                  the extent (and only to the extent) that such Derivative
                  Invention is used in connection with nucleic acid
                  hybridization assays. Chiron agrees that Gen-Probe shall have
                  exclusive rights with respect to such uses.

                           (e) Notwithstanding any other provision of this
                  Agreement, the parties agree that a Derivative Invention (as
                  defined in section 9.1.4, below) made by Chiron as to which
                  any substantial use would infringe a Valid Claim which issues
                  from [CONFIDENTIAL TREATMENT REQUESTED], shall be owned
                  jointly by the parties and considered a Joint Invention.
                  Gen-Probe waives any and all rights in any such Derivative
                  Invention to the extent (and only to the extent) that any such
                  Derivative Invention made by Chiron is used in connection with
                  immunoassays or protein binding assays, provided however that
                  nothing contained herein shall constitute a license to Chiron
                  of any rights under the [CONFIDENTIAL TREATMENT

                                       2
<PAGE>

                  REQUESTED]. Chiron waives any and all rights in any such
                  Derivative Invention to the extent (and only to the extent)
                  that such Derivative Invention is used in connection with
                  nucleic acid hybridization assays. Chiron agrees that
                  Gen-Probe shall have exclusive rights with respect to such
                  uses. This subsection (e) shall apply only to claims of the
                  [CONFIDENTIAL TREATMENT REQUESTED] when such claims have
                  issued.

                           (f) Any party granting a license of a Joint Invention
                  shall require as a term of the license that the licensee agree
                  to the restrictions set forth in Sections 9.1.1 (b), 9.1.1
                  (c), 9.1.1 (d), and 9.1.1 (e) and to include such restrictions
                  in any further licenses or sublicenses.

         4.       Section 9.1.6 of the June 11, 1998 Agreement is amended to
add, at the end thereof, the following provisions:

                  (d) Notwithstanding any other provision of this Section 9.1.6,
         the parties agree that if substantial uses of a Derivative Invention
         would infringe a Valid Claim for Core Technology of BOTH parties, then
         such Derivative Invention shall be considered as a Joint Invention
         under Section 9.1.1, PROVIDED, however, that this subsection (d) shall
         not apply to:

                  (i) a Derivative Invention as to which any substantial use
                  would infringe any Valid Claim to the same or substantially
                  the same subject matter claimed in the patents and patent
                  applications identified on Schedule "C" as items A1 through
                  A11, A16, B1 through B3, B5 through B6, C1 through C6, or D1;
                  or

                  (ii) a Derivative Invention as to which any substantial use
                  would infringe any Valid Claim which issues from the patent
                  applications identified on Schedule "C" as items A13 or A15,
                  effective immediately upon issuance or approval of such claim
                  as to Derivative Inventions made after that date. (Any license
                  of a Derivative Invention shall include such limitation.)

                  (e) The parties agree, solely for purposes of the use
         restriction of Sections 9.1.6. (a) and (b) (ii) and Sections 9.1.7 (a)
         (i) and 9.1.7 (b) (ii), that the term "Valid Claim" shall not include
         unissued claims set forth in pending patent applications pertaining to
         the Core Technology of the Disclosing Party identified on Schedules B
         and C, respectively; PROVIDED, however, that the term "Valid Claim"
         shall include a claim of such a patent application immediately upon
         issuance or approval of the claim and that any license of a Derivative
         Invention shall include such limitation. This subsection (e) shall not
         apply to exempt from the use restrictions referenced above a Derivative
         Invention as to which any substantial use would infringe any Valid
         Claim to

                                       3
<PAGE>

         the same or substantially the same subject matter claimed in the patent
         applications identified on Schedule "C" as items A9 through
         A11, B6, C5 or C6.

         5.       The parties approve the schedules attached to this First
Amendment and agree that such schedules shall serve for all purposes as
Schedules B and C to the June 11, 1998 Agreement. The parties agree that all
conditions of the Agreement In Lieu of Schedules have been satisfied and
waive the ten day limitation set forth therein.

         7.       Section 1.57 of the June 11, 1998 Agreement is amended to
read as follows:

                  "TMA Assay" shall mean an in vitro diagnostic assay based on
         or utilizing transcription-based nucleic acid amplification,
         [CONFIDENTIAL TREATMENT REQUESTED] (as that term is defined in Schedule
         E).

         8.       Article 9 of the June 11, 1998 Agreement is amended to add at
the end thereof the following provisions:

                  9.7      [CONFIDENTIAL TREATMENT REQUESTED].  Under the terms
         of the [CONFIDENTIAL TREATMENT REQUESTED], certain provisions as set
         forth in Schedule D attached hereto are incorporated into this
         Agreement by reference.

                  9.8      [CONFIDENTIAL TREATMENT REQUESTED].

         9.9 CONTINUATION OF RIGHTS. In the event the [CONFIDENTIAL TREATMENT
         REQUESTED] or [CONFIDENTIAL TREATMENT REQUESTED] is terminated for any
         reason, the rights under such agreements which are granted by Gen-Probe
         to Chiron under this Agreement, as applicable, may be continued with
         [CONFIDENTIAL TREATMENT REQUESTED] and/or [CONFIDENTIAL TREATMENT
         REQUESTED], as applicable, provided that certain conditions precedent,
         as set forth in Exhibit D, shall have been fulfilled.

         9.       The parties hereby ratify and reaffirm the June 11, 1998
Agreement in all respects, NUNC PRO TUNC, except as the Agreement is
expressly amended by this Addendum.

         By the signatures of their respective officers below, the parties adopt
this Addendum as of June 26, 1998.

CHIRON CORPORATION                       GEN-PROBE INCORPORATED

By  /s/ SEAN P. LANCE                    By   /s/ HENRY L. NORDHOFF
    -----------------                         ---------------------
        Sean P.  Lance                            Henry L.  Nordhoff
        President and                             President and
        Chief Executive Officer                   Chief Executive Officer

                                       4
<PAGE>

                                   SCHEDULE B

                               CHIRON CORPORATION
                                CORE TECHNOLOGIES

Chiron Core Technology means (1) technology claimed in Valid Claims of the
following patents and (2) technology claimed in Valid Claims of patent
applications pending before the United States Patent & Trademark Office to the
extent that such applications are substantially equivalent to the claims in the
patent applications listed below, both (1) and (2) together with any unlisted
patents and U.S. patent applications from which any of the listed patents claim
priority and any and all patents which have issued or in the future issue from
the listed patents and applications and all divisionals, continuations,
continuations-in-part, reissues, renewals, re-examinations, extensions, or
additions thereof (all only to the extent that the claims thereof are to the
same or substantially the same subject matter claimed in the listed patents or
patent applications)..

1.       HUMAN IMMUNODEFICIENCY VIRUS (HIV)

<TABLE>
<CAPTION>

----------------------------------------------------------------- -------------------- ----------------- -------------
TITLE OF INVENTION                                                PATENT/PUB. NO.      ISSUE/PUB. NO.    PRIORITY
                                                                                                         DATE
----------------------------------------------------------------------------------------------------------------------

<S>                                                               <C>                  <C>               <C>

                       [CONFIDENTIAL TREATMENT REQUESTED]

----------------------------------------------------------------------------------------------------------------------

</TABLE>

2.       HEPATITIS C VIRUS (HCV)
         -----------------------

<TABLE>
<CAPTION>

----------------------------------------------------------------- -------------------- ----------------- -------------
TITLE OF INVENTION                                                PATENT/PUB.  NO.     ISSUE/PUB.  NO.   PRIORITY
                                                                                                         DATE
----------------------------------------------------------------------------------------------------------------------

<S>                                                               <C>                  <C>               <C>
                       [CONFIDENTIAL TREATMENT REQUESTED]

----------------------------------------------------------------------------------------------------------------------

</TABLE>

3.       [CONFIDENTIAL TREATMENT REQUESTED]

<TABLE>
<CAPTION>

----------------------------------------------------------------- -------------------- ----------------- -------------
TITLE OF INVENTION                                                PATENT/PUB.  NO.     ISSUE/PUB.  NO.   PRIORITY
                                                                                                         DATE
----------------------------------------------------------------------------------------------------------------------

<S>                                                               <C>                  <C>               <C>

                       [CONFIDENTIAL TREATMENT REQUESTED]

----------------------------------------------------------------------------------------------------------------------

</TABLE>

4.       [CONFIDENTIAL TREATMENT REQUESTED]

<TABLE>
<CAPTION>

----------------------------------------------------------------- -------------------- ----------------- -------------
TITLE OF INVENTION                                                PATENT/PUB.  NO.     ISSUE/PUB.  NO.   PRIORITY
                                                                                                         DATE
----------------------------------------------------------------------------------------------------------------------

<S>                                                               <C>                  <C>               <C>

                       [CONFIDENTIAL TREATMENT REQUESTED]

----------------------------------------------------------------------------------------------------------------------

</TABLE>

                                  Page 1 of 3
<PAGE>

                                   SCHEDULE C

                             GEN-PROBE INCORPORATED
                                CORE TECHNOLOGIES

     Gen-Probe Core Technologies means (1) technology claimed in Valid Claims of
the following patents and (2) technology claimed in Valid Claims of patent
applications pending before the United States Patent & Trademark Office to the
extent that such applications are substantially equivalent to the claims in the
patent applications listed below, both (1) and (2) together with any unlisted
patents and the U.S. patent applications from which any of the listed patents
claim priority and any and all patents which have issued or in the future issue
from the listed patents and applications and all divisionals, continuations,
continuations-in-part, reissues, renewals, re-examinations, extensions, or
additions thereof (all only to the extent that the claims thereof are to the
same or substantially the same subject matter claimed in the listed patents or
patent applications).

A.       [CONFIDENTIAL TREATMENT REQUESTED]

<TABLE>
<CAPTION>

                  Title of Invention               Issue/Pub.No       Issue/Pub.      Priority
                                                                           Date           Date
-----------------------------------------------------------------------------------------------------
<S>                                               <C>               <C>              <C>

                                  [CONFIDENTIAL TREATMENT REQUESTED]

-----------------------------------------------------------------------------------------------------

</TABLE>

B.       [CONFIDENTIAL TREATMENT REQUESTED]

<TABLE>
<CAPTION>

-----------------------------------------------------------------------------------------------------

<S>                                               <C>               <C>              <C>

                                  [CONFIDENTIAL TREATMENT REQUESTED]

-----------------------------------------------------------------------------------------------------

</TABLE>

C.       [CONFIDENTIAL TREATMENT REQUESTED]

     [CONFIDENTIAL TREATMENT REQUESTED].

<TABLE>
<CAPTION>

-----------------------------------------------------------------------------------------------------

<S>                                               <C>               <C>              <C>

                                  [CONFIDENTIAL TREATMENT REQUESTED]

-----------------------------------------------------------------------------------------------------

</TABLE>

D.       [CONFIDENTIAL TREATMENT REQUESTED]

<TABLE>
<CAPTION>

-----------------------------------------------------------------------------------------------------

<S>                                               <C>               <C>              <C>

                                  [CONFIDENTIAL TREATMENT REQUESTED]

-----------------------------------------------------------------------------------------------------

</TABLE>

                                     Page 1
<PAGE>

E.       [CONFIDENTIAL TREATMENT REQUESTED]

<TABLE>
<CAPTION>

-----------------------------------------------------------------------------------------------------

<S>                                               <C>               <C>              <C>

                                  [CONFIDENTIAL TREATMENT REQUESTED]

-----------------------------------------------------------------------------------------------------

</TABLE>

F.       [CONFIDENTIAL TREATMENT REQUESTED]

     [CONFIDENTIAL TREATMENT REQUESTED].

<TABLE>
<CAPTION>

-----------------------------------------------------------------------------------------------------

<S>                                               <C>               <C>              <C>

                                  [CONFIDENTIAL TREATMENT REQUESTED]

-----------------------------------------------------------------------------------------------------

</TABLE>

                                     Page 2

<PAGE>

                                   SCHEDULE D

      ADDITIONAL CLAUSES RELATING TO THE [CONFIDENTIAL TREATMENT REQUESTED]
                   AND THE [CONFIDENTIAL TREATMENT REQUESTED]

[CONFIDENTIAL TREATMENT REQUESTED]

         The [CONFIDENTIAL TREATMENT REQUESTED] sets forth the following terms
and conditions relating to required provisions in any sublicense and to the
continuation of sublicenses following termination of the [CONFIDENTIAL TREATMENT
REQUESTED].

         4.3.2 Any such sublicense shall also expressly include the provisions
of Article 7, 8, and 9 for the benefit of [CONFIDENTIAL TREATMENT REQUESTED].

         4.3.3 Upon termination of this Agreement for any cause, any sublicense
granted hereunder shall continue with [CONFIDENTIAL TREATMENT REQUESTED]
provided the sublicensee agrees to thereafter assume the obligations of
GEN-PROBE insofar as they correspond to the scope of the sublicense.

7.       REPORTS, PAYMENTS, AND ACCOUNTING

         7.1 Quarterly Royalty Payment and Report. GEN-PROBE shall make written
reports and royalty payments to [CONFIDENTIAL TREATMENT REQUESTED] within ninety
(90) days after the end of each calendar quarter. This report shall state the
number, description, and aggregate Net Sales of Licensed Product(s) during such
completed calendar quarter, and resulting calculation pursuant to Paragraph 6.2
of earned royalty payment due [CONFIDENTIAL TREATMENT REQUESTED] for such
completed calendar quarter. Concurrent with the making of each such report,
GEN-PROBE shall include payment due [CONFIDENTIAL TREATMENT REQUESTED] of
royalties for the calendar quarter covered by such report.

         7.2 Accounting. GEN-PROBE agrees to keep records for a period of three
(3) years showing the manufacturing, sales, use, and other disposition of
products sold or otherwise disposed of under the license herein granted in
sufficient detail to enable the royalties payable hereunder by GEN-PROBE to be
determined, and further agrees to permit its books and records to be examined to
the extent necessary to verify reports provided for in Paragraph 7.1 by an
independent certified public accountant, provided that such audits occur no more
than one (1) time per calendar year and provided further that accountant shall
report to [CONFIDENTIAL TREATMENT REQUESTED] only errors regarding calculation
of royalties. Such examination is to be made by [CONFIDENTIAL TREATMENT
REQUESTED], at the expense of [CONFIDENTIAL TREATMENT REQUESTED], except in the
event that the results

                                       1
<PAGE>

of the audit reveal a discrepancy in GEN-PROBE's favor of ten percent (10%) or
more, then the audit fees shall be paid by GEN-PROBE.

8.       WARRANTY AND NEGATION OF WARRANTIES

         8.1 Nothing in this Agreement is or shall be construed as:

         (a) A warranty or representation by [CONFIDENTIAL TREATMENT REQUESTED]
as to the validity or scope of any Licensed Patent(s);

         (b) A warranty or representation that anything made, used, sold, or
otherwise disposed of under any license granted in this Agreement is or will be
free from of patents, copyrights, and other rights of third parties;

         (c) An obligation to bring or prosecute actions or suits against third
parties for the infringement, except to the extent and in the circumstances
described in Article 12; or

         (d) Granting by implication, estoppel, or otherwise any licenses under
patents of [CONFIDENTIAL TREATMENT REQUESTED] or other persons other than
Licensed Patent(s), regardless of whether such patents are dominant or
subordinate to any Licensed Patent(s)

         8.2 EXCEPT AS EXPRESSLY SET FORTH IN THIS AGREEMENT, STANFORD MAKES NO
REPRESENTATIONS AND EXTENDS NO WARRANTIES OF ANY KIND, EITHER EXPRESS OR
IMPLIED. THERE ARE NO EXPRESS OR IMPLIED WARRANTIES OF MERCHANTABILITY OR
FITNESS FOR A PARTICULAR PURPOSE, OR THAT THE USE OF THE LICENSED PRODUCT(S)
WILL NOT INFRINGE ANY PATENT, COPYRIGHT, TRADEMARK, OR OTHER RIGHTS OF THIRD
PARTIES.

9.       INDEMNITY

         9.1 GEN-PROBE agrees to indemnify, hold harmless, and defend
[CONFIDENTIAL TREATMENT REQUESTED] and its trustees, officers, employees,
students, and agents against any and all claims for death, illness, personal
injury, property damage, and improper business practices arising out of the
manufacture, use, sale, or other disposition of Invention, Licensed Patent(s),
or Licensed Product(s) by GEN-PROBE, its sublicenses or their customers.

         9.2 GEN-PROBE shall at all times comply, through insurance or
self-insurance, with all statutory workers' compensation and employers'
liability requirements covering any and all employees with respect to activities
performed under this Agreement.

         9.3 GEN-PROBE shall maintain, beginning on the first day GEN-PROBE or
any GEN-PROBE sublicensee ships a therapeutics product manufactured by the
licensed

                                       2
<PAGE>

process, commercial general liability insurance, including products
liability insurance, with minimum limits of liability of $5 million reputable
and financially secure insurance carrier(s) to cover the activities of GEN-PROBE
and its sublicensee(s). Any and all such policies of insurance described in the
previous sentence shall include as additional named insureds [CONFIDENTIAL
TREATMENT REQUESTED] and shall provide that such policies may not, without 30
days prior written notice to [CONFIDENTIAL TREATMENT REQUESTED] be canceled or
changed to materially adversely affect any such additional named insured's
coverage. Such insurance shall be written to cover liability of such additional
named insureds incurred beginning on the first day GEN-PROBE or any GEN-PROBE
sublicensee ships a therapeutics product.

[CONFIDENTIAL TREATMENT REQUESTED]

         The [CONFIDENTIAL TREATMENT REQUESTED] sets forth the following sets
forth the following terms and conditions relating to rights of Gen-Probe's
Licensees should a breach by Gen-Probe terminate its rights under the
[CONFIDENTIAL TREATMENT REQUESTED]

         10.3 Upon termination of this Agreement, as permitted by Section 10.2,
the non-breaching party shall grant direct immunity from legal action to
Licensees of the breaching party, under the terms and conditions set forth in
this Agreement, provided that:

         (a) the non-breaching party shall have received express written notice
of the license granted to such Licensee prior to the effective termination date
of this Agreement;

         (b) the Licensee expressly agreed in writing on or before the ninetieth
(90th) day prior to the effective termination date of this Agreement to be bound
by the terms and conditions of this Agreement.

                                       3
<PAGE>

                                    EXHIBIT E

                   [CONFIDENTIAL TREATMENT REQUESTED] VERSIONS

         "GEN-PROBE Version" shall mean [CONFIDENTIAL TREATMENT REQUESTED]

              (a) [CONFIDENTIAL TREATMENT REQUESTED];

              (b) [CONFIDENTIAL TREATMENT REQUESTED]

              (c) [CONFIDENTIAL TREATMENT REQUESTED].

          "[CONFIDENTIAL TREATMENT REQUESTED]

              (a) [CONFIDENTIAL TREATMENT REQUESTED];

              (b) [CONFIDENTIAL TREATMENT REQUESTED]

              (c) [CONFIDENTIAL TREATMENT REQUESTED].

[CONFIDENTIAL TREATMENT REQUESTED]

[CONFIDENTIAL TREATMENT REQUESTED]

[CONFIDENTIAL TREATMENT REQUESTED].

[CONFIDENTIAL TREATMENT REQUESTED]:

[CONFIDENTIAL TREATMENT REQUESTED]
[CONFIDENTIAL TREATMENT REQUESTED]
[CONFIDENTIAL TREATMENT REQUESTED]
[CONFIDENTIAL TREATMENT REQUESTED]
[CONFIDENTIAL TREATMENT REQUESTED]

[CONFIDENTIAL TREATMENT REQUESTED].

[CONFIDENTIAL TREATMENT REQUESTED]:

[CONFIDENTIAL TREATMENT REQUESTED]
[CONFIDENTIAL TREATMENT REQUESTED]
[CONFIDENTIAL TREATMENT REQUESTED]
[CONFIDENTIAL TREATMENT REQUESTED]
[CONFIDENTIAL TREATMENT REQUESTED]

                                       1
<PAGE>

[CONFIDENTIAL TREATMENT REQUESTED]

[CONFIDENTIAL TREATMENT REQUESTED]

[CONFIDENTIAL TREATMENT REQUESTED]

[CONFIDENTIAL TREATMENT REQUESTED]

[CONFIDENTIAL TREATMENT REQUESTED]
[CONFIDENTIAL TREATMENT REQUESTED]
[CONFIDENTIAL TREATMENT REQUESTED]
[CONFIDENTIAL TREATMENT REQUESTED]
[CONFIDENTIAL TREATMENT REQUESTED]
[CONFIDENTIAL TREATMENT REQUESTED]
[CONFIDENTIAL TREATMENT REQUESTED]

[CONFIDENTIAL TREATMENT REQUESTED]

Equivalent unit definitions and assay conditions may be substituted as long as
they produce results that may be quantitatively converted by established valid
procedures (e.g., multiplication by a conversion factor or by reference to a
standard curve) to the assay conditions and unit definitions set forth above.

For example, [CONFIDENTIAL TREATMENT REQUESTED]

Data supporting equivalence of any substitutions must be made available for
inspection as set forth in Section 3.3 of this Agreement.

                                       2<PAGE>

                                                                        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].

<PAGE>

         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

                                  Page 2 of 2

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