EXHIBIT 99.4

                                    GUARANTEE


         This GUARANTEE ("Guarantee") is made and entered into as of this 20th
day of February, 2003, in favor of AMERICAN PHARMACEUTICAL PARTNERS, INC., a
Delaware corporation ("APP"), with offices at 1101 Perimeter Drive, Suite 300,
Schaumburg, Illinois 60173-5837, USA, by JOHN TRAMONTANA, an individual
("Guarantor") with residences at 10890 Camp Ohio Road NE, Utica, Ohio 43080
(Ohio residence) and Riva Caccia 1d, CH-6900 Lugano, Switzerland (Lugano
residence).

         WHEREAS, Tramontana is Chairman, President and Chief Executive Officer,
as well as a principal shareholder, of Bigmar, Inc., a Delaware corporation,
which, in turn, owns all of the shares in Bigmar Pharmaceuticals, SA, a Swiss
corporation (collectively, "Bigmar"); and

         WHEREAS, APP has advanced and may in the future advance certain funds
to Bigmar in respect of Bigmar's purchases of methotrexate active pharmaceutical
ingredient for Bigmar's conversion into finished pharmaceutical products for
sale to APP (hereinafter, the "Advance"); and

         WHEREAS, the Advance has been memorialized in an Advance Agreement
between APP and Bigmar Pharmaceuticals, SA (with Bigmar, Inc. as a guarantor to
its subsidiary's obligations), which Advance Agreement provides for APP to be
repaid first through credits against invoices for purchases of the finished
product and then through repayment by Bigmar if full repayment is not achieved
through such credits within eighteen (18) months of the date of the Advance
Agreement; and

         WHEREAS, APP and Guarantor have agreed that Guarantor shall provide a
personal guarantee of Bigmar's obligations under the Advance Agreement as an
additional safeguard to the repayment to APP of the Advance.

         NOW, THEREFORE, Guarantor hereby covenants and agrees as follows:

         1.       Guarantor hereby absolutely and unconditionally guarantees to
                  APP the due and punctual payment in full, in lawful money of
                  the United States, of any and all sums which may at any time
                  be or become due and payable under and pursuant to the Advance
                  Agreement (a copy of which is attached hereto), as and when
                  the same shall be due and payable, whether by lapse of time or
                  otherwise.

         2.       Guarantor hereby agrees that its obligations hereunder are an
                  unconditional and absolute guarantee of payment of the terms
                  and provisions of the Advance Agreement, and shall remain in
                  full force and effect, irrespective of any waiver, consent, or
                  granting of any indulgence of APP or any other person to
                  Bigmar (either the parent or the subsidiary) with respect to
                  any provision of the Advance Agreement, irrespective of
                  whether APP has instituted any suit, action, or proceeding or
                  exhausted its remedies under the Advance Agreement or taken
                  any steps to enforce any rights against Bigmar (either the
                  parent or the subsidiary) to compel such performance or to
                  collect all or part of any payment of the Advance, at law or
                  in equity, irrespective of whether APP or any other person
                  shall have recovered any judgment against Bigmar (either the
                  parent or the subsidiary), and irrespective of any other
                  circumstances or contingencies.

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         3.       Guarantor hereby waives diligence, presentment, demand of
                  payment, filing of claims with a court in the event of merger
                  or bankruptcy of Bigmar (either the parent or the subsidiary),
                  any right to require a proceeding first against Bigmar (either
                  the parent or the subsidiary) of any other person, protest,
                  notice of default in the payment of any sum payable by Bigmar
                  (either the parent or the subsidiary) under the Advance
                  Agreement, notice of any other default, breach or
                  nonperformance of any agreement, covenant or obligation of
                  Bigmar (either the parent or the subsidiary) under the Advance
                  Agreement, notice and all demands whatsoever, with respect to
                  the Advance Agreement or any indebtedness evidenced thereby.

         4.       Guarantor hereby expressly waives notice from APP of its
                  acceptance of and reliance on this Guarantee. Guarantor agrees
                  to pay all costs, expenses and fees, including reasonable
                  attorneys fees and expenses, which may be incurred in
                  enforcing or attempting to enforce this Guarantee following
                  any default on the part of Guarantor, whether the same shall
                  be enforced by suit or otherwise.

         5.       No amendment, release or modification of the provisions of
                  this Guarantee shall be established by conduct, custom or
                  course of dealing, by solely by an instrument in writing duly
                  signed by APP and Guarantor. No delay or omission by APP to
                  exercise any right under this Guarantee shall impair any such
                  right, nor shall it be construed to be a waiver thereof.

         6.       The obligations of Guarantor under this Guarantee shall not be
                  altered, limited, or affected by any proceeding, voluntary or
                  involuntary, involving the bankruptcy, insolvency,
                  receivership, reorganization, liquidation or arrangement of
                  Bigmar (either the parent or the subsidiary), or any defense
                  Bigmar (either the parent or the subsidiary) may have by
                  reason of the order or decision of any court or administrative
                  body resulting from such proceeding.

         7.       Guarantor represents and warrants that he is Chairman,
                  President and Chief Executive Officer, as well as a principal
                  shareholder, of Bigmar, Inc., a Delaware corporation, and that
                  Bigmar, Inc. owns all of the shares in Bigmar Pharmaceuticals,
                  SA, a Swiss corporation (collectively, "Bigmar"). Guarantor
                  further represents and agrees that the consideration received
                  for this Guarantee is reasonably worth at least as much as the
                  liability and obligation under this Guarantee.

         8.       This Guarantee shall be governed by and construed in
                  accordance with the laws of the State of Illinois.

Executed this 27th day of February, 2003.


/s/John Tramontana
- ------------------------------------
John Tramontana
Guarantor


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