<PAGE>

                                                                 Exhibit 10.102

                                   GUARANTY
                       CHIRON REGARDING CETUS DEBENTURES


              THIS GUARANTY (this "Guaranty"), dated as of September 29,
1994, is made by Chiron Corporation, a Delaware corporation (the
"Guarantor"), in favor of Bankers Trust Company, as trustee (the "Trustee"),
for the benefit of the holders (the "Holders") of the 5 1/4% Convertible
Subordinated Debentures due 2000 (the "Debentures") of Cetus Corporation, a
Delaware corporation ("Cetus"), issued pursuant to an Indenture dated as of
May 21, 1987 between Cetus and the Trustee (as amended, the "Indenture").

              Cetus has requested that the Guarantor execute and deliver this
Guaranty in favor of the Trustee for the benefit of the Holders and the
Guarantor derives substantial direct and indirect benefits from the
Debentures issued by Cetus to the Holders (which benefits are hereby
acknowledged by the Guarantor).

              Accordingly, the Guarantor hereby agrees as follows:

              SECTION 1 DEFINITIONS.  All capitalized terms used in this
Guaranty and not otherwise defined herein shall have the meanings assigned to
them in the Indenture.

              SECTION 2 GUARANTY.  The Guarantor hereby unconditionally and
irrevocably guarantees to the Trustee, for the benefit of the Holders, and
their respective successors and assigns, the full and prompt payment when due
(whether at stated maturity, by required prepayment, declaration,
acceleration, demand, or otherwise) and performance of the indebtedness,
liabilities and other obligations of Cetus to the Trustee for the benefit of
the Holders under the Indenture and the Debentures.  The foregoing
indebtedness, liabilities and other obligations of Cetus, and all other
indebtedness, liabilities and obligations to be paid or performed by the
Guarantor in connection with this Guaranty shall hereinafter be collectively
referred to as the "Guaranteed Obligations."

              SECTION 3 LIABILITY OF GUARANTOR.  The liability of the
Guarantor under this Guaranty shall be irrevocable, absolute, independent and
unconditional, and shall not be affected by any circumstance which might
constitute a discharge of a surety or guarantor other than the indefeasible
payment and performance in full of all Guaranteed Obligations.  In
furtherance of the foregoing and without limiting the generality thereof, the
Guarantor agrees as follows:

                      (i) the Guarantor's liability hereunder shall be the
immediate, direct, and primary obligation of the Guarantor and shall not be
contingent upon the Trustee's exercise or enforcement of any remedy it may
have against Cetus or any other Person;


                                      1.

<PAGE>

                     (ii) this Guaranty is a guaranty of payment when due and
not merely of collectibility;

                    (iii) the Guarantor's payment of a portion, but not all,
of the Guaranteed Obligations shall in no way limit, affect, modify or
abridge the Guarantor's liability for any portion of the Guaranteed
Obligations remaining unsatisfied; and

                     (iv) the Guarantor's liability with respect to the
Guaranteed Obligations shall remain in full force and effect without regard
to, and shall not be impaired or affected by, nor shall the Guarantor be
exonerated or discharged by, any of the following events:

                        (A)  any insolvency, bankruptcy, reorganization,
arrangement, adjustment, composition, assignment for the benefit of
creditors, liquidation, winding up or dissolution of Cetus or the Guarantor;

                        (B)  any limitation, discharge, or cessation of the
liability of Cetus, the Guarantor, any other guarantor or any other Person
for any Guaranteed Obligations due to any statute, regulation or rule of law,
or any invalidity or unenforceability in whole or in part of any of the
Guaranteed Obligations, the Indenture or the Debentures;

                        (C)  any merger, acquisition, consolidation or change
in structure of Cetus, the Guarantor or any other Person, or any sale, lease,
transfer or other disposition of any or all of the assets or shares of Cetus,
the Guarantor, or any other Person;

                        (D)  any claim, counterclaim or setoff, other than
that of prior performance, that Cetus, the Guarantor, any other guarantor or
other Person may have or assert;

                        (E)  the Trustee's amendment, modification, renewal,
extension or cancellation of the Indenture, the Debentures or any Guaranteed
Obligations;

                        (F)  any assignment or other transfer, in whole or in
part, of the Trustee's interests in and rights under this Guaranty, the
Indenture or the Debentures; and

                        (G)  the Trustee's vote, claim, distribution,
election, or acceptance, in any bankruptcy case related to the Guaranteed
Obligations.


                                      2.

<PAGE>

              SECTION 4 CONSENTS OF GUARANTOR.  The Guarantor hereby
unconditionally consents and agrees that, without notice to or further assent
from the Guarantor:

                   (i)  the principal amount of the Guaranteed Obligations
may be increased or decreased and additional indebtedness or obligations of
Cetus under the Indenture may be incurred, by one or more amendments,
modifications, renewals or extensions of the Indenture, the Debentures or
otherwise;

                  (ii)  the time, manner, place or terms of any payment under
the Indenture or the Debentures may be extended or changed, including by an
increase or decrease in the interest rate on any Guaranteed Obligation or
other amount payable under the Indenture or the Debentures, by an amendment,
modification or renewal of the Indenture or the Debentures;

                 (iii)  the time for Cetus's performance of or compliance
with any term, covenant or agreement on its part to be performed or observed
under the Indenture may be extended, or such performance or compliance
waived, or failure in or departure from such performance or compliance
consented to, all in such manner and upon such terms as the Trustee may deem
proper;

all without impairing, abridging, or releasing this Guaranty;

                  (iv)  the Trustee may discharge or release, in whole or in
part, any other guarantor or any other Person liable for the payment and
performance of all or any part of the Guaranteed Obligations, may permit or
consent to any such action or any result of such action, and the Trustee
shall not be liable to the Guarantor for any failure to collect or enforce
payment or performance of the Guaranteed Obligations from any Person;

                   (v)  the Trustee may request and accept other guaranties
of the Guaranteed Obligations and any other indebtedness, obligations or
liabilities of Cetus to the Trustee and may, from time to time, in whole or
in part, surrender, release, subordinate, modify, waive, rescind, compromise
or extend any such guaranty and may permit or consent to any such action or
the result of any such action; and

                  (vi)  the Trustee may exercise, or waive or otherwise
refrain from exercising, any other right, remedy, power or privilege granted
by any document or agreement, or otherwise available to the Trustee, with
respect to the Guaranteed Obligations, even if the exercise of such right,
remedy, power or privilege affects or eliminates any right of subrogation or
any other right of the Guarantor against Cetus;

                                      3.
<PAGE>

              SECTION 5 GUARANTOR'S WAIVERS.

              (a)  CERTAIN WAIVERS.  The Guarantor waives and agrees not to
assert:

                   (i)  any right to require the Trustee to marshal assets in
favor of Cetus, the Guarantor, any other guarantor or any other Person, to
proceed against Cetus, any other guarantor or any other Person, or to pursue
any other right, remedy, power or privilege of the Trustee whatsoever;

                  (ii)  the defense of the statute of limitations in any
action hereunder or for the collection or performance of the Guaranteed
Obligations;

                 (iii)  any defense arising by reason of any lack of
corporate or other authority;

                  (iv)  any rights to set-offs and counterclaims;

                   (v)  any defense based upon an election of remedies which
destroys or impairs the subrogation rights of the Guarantor or the right of
the Guarantor to proceed against Cetus or any other obligor of the Guaranteed
Obligations for reimbursement; and

                  (vi) without limiting the generality of the foregoing, to
the fullest extent permitted by law, any defenses or benefits that may be
derived from or afforded by applicable law limiting the liability of or
exonerating guarantors or sureties, or which may conflict with the terms of
this Guaranty.

              (b)  ADDITIONAL WAIVERS.  The Guarantor waives any and all
notice of the acceptance of this Guaranty, and any and all notice of the
creation, renewal, modification, extension or accrual of the Guaranteed
Obligations, or the reliance by the Trustee upon this Guaranty, or the
exercise of any right, power or privilege hereunder. The Guaranteed
Obligations shall conclusively be deemed to have been created, contracted,
incurred and permitted to exist in reliance upon this Guaranty. The Guarantor
waives promptness, diligence, presentment, protest, demand for payment,
notice of default, dishonor or nonpayment and all other notices to or upon
Cetus, the Guarantor or any other Person with respect to the Guaranteed
Obligations.

              (c)  INDEPENDENT OBLIGATIONS.  The obligations of the Guarantor
hereunder are independent of and separate from the obligations of Cetus and
any other guarantor and upon the occurrence and during the continuance of any
Event of Default, a separate action or actions may be brought against the
Guarantor, whether or not Cetus or any such other guarantor is joined therein
or a separate action or actions are brought against Cetus or any such other
guarantor.

                                      4.

<PAGE>

              SECTION 6 SUBROGATION.

              So long as the Guaranteed Obligations remain unpaid in full,
the Guarantor shall not have, and shall not directly or indirectly exercise,
(i) any rights that it may acquire by way of subrogation under this Guaranty,
by any payment hereunder or otherwise, (ii) any rights of contribution,
indemnification, reimbursement or similar suretyship claims arising out of
this Guaranty or (iii) any other right which it might otherwise have or
acquire (in any way whatsoever) which could entitle it at any time to share
or participate in any right, remedy or security of the Trustee as against
Cetus or other guarantors in connection with this Guaranty, the Indenture or
any of the Debentures. If any amount shall be paid to the Guarantor on
account of the foregoing rights at any time when all the Guaranteed
Obligations shall not have been paid in full, such amount shall be held in
trust for the benefit of the Trustee and shall forthwith be paid to the
Trustee to be credited and applied to the Guaranteed Obligations, whether
matured or unmatured, in accordance with the terms of the Indenture and the
Debentures.

              SECTION 7 CONTINUING GUARANTY; REINSTATEMENT.

              (a)  CONTINUING GUARANTY. This Guaranty is a continuing
guaranty relating to any Guaranteed Obligations, including Guaranteed
Obligations which may exist continuously or which may arise from time to time
under successive transactions, and the Guarantor expressly acknowledges that
this Guaranty shall remain in full force and effect notwithstanding that
there may be periods in which no Guaranteed Obligations exist. This Guaranty
shall continue in effect and be binding upon the Guarantor until actual
receipt by the Trustee of written notice from the Guarantor of its intention
to discontinue this Guaranty as to future transactions (which notice shall
not be effective until noon on the day which is ten days following such
receipt); PROVIDED that no revocation or termination of this Guaranty shall
affect in any way any rights of the Trustee hereunder with respect to any
Guaranteed Obligations arising or outstanding on the date of receipt of such
notice, including any subsequent continuation, extension, or renewal thereof,
or change in the terms or conditions thereof (collectively, "Existing
Guaranteed Obligations"), and the sole effect of such notice shall be to
exclude from this Guaranty Guaranteed Obligations thereafter arising which
are unconnected to any Existing Guaranteed Obligations.

              (b)  REINSTATEMENT. This Guaranty shall continue to be
effective or shall be reinstated and revived, as the case may be, if, for any
reason, any payment of the Guaranteed Obligations by or on behalf of Cetus
shall be rescinded, invalidated, declared to be fraudulent or preferential,
set aside, voided or otherwise required to be repaid to Cetus, its estate,
trustee, receiver or any other Person, or must otherwise be restored by the
Trustee, whether as a result of proceedings in bankruptcy or reorganization
or otherwise. To the extent any payment is so rescinded, set aside, voided

                                       5.
<PAGE>

or otherwise repaid or restored, the Guaranteed Obligations shall be revived
in full force and effect without reduction or discharge for such payment.

              SECTION 8 PAYMENTS. The Guarantor hereby agrees, in furtherance
of the foregoing provisions of this Guaranty and not in limitation of any
other right which the Trustee or any other Person may have against the
Guarantor by virtue hereof, upon the failure of Cetus to pay any of the
Guaranteed Obligations when and as the same shall become due, whether at
stated maturity, by required prepayment, declaration, acceleration, demand or
otherwise (including amounts that would become due but for the operation of
the automatic stay under Section 362(a) of Title 11 of the United States
Bankruptcy Code, as amended), the Guarantor shall forthwith pay, or cause to
be paid, in cash, to the Trustee an amount equal to the amount of the
Guaranteed Obligations then due as aforesaid (including interest which, but
for the filing of a petition in bankruptcy with respect to Cetus, would have
accrued on such Guaranteed Obligations, whether or not a claim is allowed
against Cetus for such interest in any such bankruptcy proceeding). The
Guarantor shall make each payment hereunder, unconditionally in full without
set-off or counterclaim, or deduction for any taxes, on the day when due, to
the Trustee at such office of the Trustee and to such account as the Trustee
shall request.

              SECTION 9 GOVERNING LAW. THIS GUARANTY SHALL BE GOVERNED BY,
AND CONSTRUED IN ACCORDANCE WITH, THE LAW OF THE STATE OF NEW YORK.

              SECTION 10 ENTIRE AGREEMENT; AMENDMENTS AND WAIVERS.

              (a) ENTIRE AGREEMENT. This Guaranty constitutes the entire
agreement of the Guarantor with respect to the matters set forth herein and
supersedes any prior agreements, commitments, drafts, communications,
discussions and understandings, oral or written, with respect thereto.

              (b) AMENDMENTS. This Guaranty may not be amended except by a
writing signed by the Guarantor.

              SECTION 11 SEVERABILITY. Whenever possible, each provision of
this Guaranty shall be interpreted in such manner as to be effective and
valid under all applicable laws and regulations. If, however, any provision
of this Guaranty shall be prohibited by or invalid under any such law or
regulation in any jurisdiction, it shall, as to such jurisdiction, be deemed
modified to conform to the minimum requirements of such law or regulation,
or, if for any reason it is not deemed so modified, it shall be ineffective
and invalid only to the extent of such prohibition or invalidity without
affecting the remaining provisions of this Guaranty, or the validity or
effectiveness of such provision in any other jurisdiction.

                                       6.

<PAGE>

              IN WITNESS WHEREOF, the Guarantor has executed this Guaranty,
as of the date first above written.

                                          CHIRON CORPORATION

                                          By: /s/ Dennis L. Winger
                                              ---------------------------
                                          Title: Senior V.P. & CFO

                                          Address:

                                          1400 Fifty-Third Street
                                          Emeryville, California 94608
                                          Attn: Chief Financial Officer
                                          Fax No.: (510) 601-3343




                                        7.