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                                  Exhibit 4.3
                       FIRST AMENDMENT TO RIGHTS AGREEMENT
                       -----------------------------------

         This FIRST AMENDMENT TO RIGHTS AGREEMENT is made as of September 30,
1999 by and between Hemagen Diagnostics, Inc. (the "Company"), a Delaware
corporation, and Continental Stock Transfer & Trust Company (the "Rights
Agent"), a New York corporation.

         WHEREAS, the Company and the Rights Agent entered into a Rights
Agreement dated as of January 27, 1999 (the "Rights Agreement");

         WHEREAS, Section 28 of the Rights Agreement provides that the Company
and the Rights Agent may amend any provision of the Rights Agreement without
shareholder approval subject to certain limitations; and

         WHEREAS, the Company and the Rights Agent desire to amend the Rights
Agreement as provided herein.

         NOW, THEREFORE, the parties hereby agree as follows:

         1.       Section 1(a)(i) of the Rights Agreement shall be superseded,
                  amended and restated as follows:

                  "(i) none of Jerry L. Ruyan, Thomas A. Donelan, William P.
                  Hales, Christopher P. Hendy or Redwood Holdings, Inc. shall be
                  deemed to be an "Acquiring Person" and".

         1.       This First Amendment to Rights Agreement may be executed in
                  counterparts, and each such counterpart shall be deemed to be
                  an original instrument, but all such counterparts together
                  shall constitute but one agreement.

         IN WITNESS WHEREOF, the parties hereto have caused this First Amendment
to Rights Agreement to be duly executed as of the date first above written.

                                         HEMAGEN DIAGNOSTICS, INC.

                                         By: /s/ WILLIAM P. HALES
                                            --------------------------------
                                                Name: William P. Hales
                                                Title: President

                                         CONTINENTAL STOCK TRANSFER
                                         & TRUST COMPANY,
                                         As Rights Agent

                                         By: /s/ WILLIAM F. SEEGRABER
                                            --------------------------------
                                                Name: William F. Seegraber
                                                Title: Vice President<PAGE>   1

                                 Exhibit 10.30
                           SECOND AMENDMENT TO LEASE

         Hemagen Diagnostics, Inc., a Delaware corporation (hereinafter referred
to as "Tenant"), entered into a Lease (hereinafter referred to as the "Lease")
with the then owner of the 9033 Red Branch Road building in Columbia, Maryland,
for a space containing approximately 27,400 square feet of gross floor area.
Thereafter, the then owner of the building, E. Fulton Brylawski, conveyed his
interest in the 9033 Red Branch Road building to the 9033 Red Branch Road,
L.L.C. on December 30, 1997. The then existing Lease was modified to provide
that the Lease term to be extended for an additional two (2) years commencing on
July 1, 2000 and expiring on June 30, 2002. The Lease was further modified to
provide that the rent from the period July 1, 2000 to June 30, 2001 would be
$4.95 per square foot, Absolute Net based on 27,400 square feet and that the
rental rate from the time period July 1, 2001 - June 30, 2002 would be $5.25 per
square foot, Absolute Net based on 27,400 of leased area. The Tenant further
agreed in the First Lease Amendment to pay a maximum management fee of $.10 per
square foot of leased area per year. Except as amended by that First Amendment
to the Lease, the original Lease commencing on July 1, 1995 remained unmodified
and in full force and effect.

The Landlord and the Tenant now wish to further amend the existing Lease with a
Second Amendment To Lease. In consideration of the mutual promises and other
consideration already received, receipt of which is acknowledged by each party
to this Amendment, Landlord and Tenant hereby agree as follows:

1.    Paragraph 2 of the Lease shall be modified such that the Lease term is
      extended for seven (7) years, commencing on July 1, 2000, and expiring on
      June 30, 2007.

2.    Paragraph 3 of the Lease shall be modified to the extent that the rent
      from the time period July 1, 2000 - June 30, 2001 shall be $4.95 per
      square foot Absolute Net based on 27,400 square feet. Thereafter the rent
      shall be as follows:

      July 1, 2001 - June 30,2004:        $5.25/sq. ft., absolute Net ($143,850
                                       per year absolute Net)

      July 1, 2004 - June 30, 2005:       $5.50/sq. ft., absolute Net ($150,700
                                       per year absolute Net)

      July 1, 2005 - June 30, 2006:       $5.75/sq. ft., absolute Net ($157,550
                                       per year absolute Net)

      July 1, 2006 - June 30, 2007:       $6.00/sq. ft., absolute Net ($164,400
                                       per year absolute Net)

      Except as amended by the Second Amendment to the Lease, the original Lease
      commencing on July 1, 1995 shall remain in full force and effect.

3.    Paragraph 3.3 of the Lease shall be amended by changing the last sentence
      of the existing paragraph to read "a maximum management fee equal to $.15
      per square foot of the leased area" in lieu of the $.10 per square foot
      currently provided in the First Amendment To Lease. This change will be
      effective July 1, 2002.

<PAGE>   2

4.    Tenant may reduce the rent payment for the month of December 2000 for a
      maximum of $5,000.00 based on carpet and painting expenses incurred in
      calendar 2000.

5.    Tenant, by giving Landlord notice no later then December 31, 2006, shall
      have the right to extend this lease for one (1) additional term of five
      (5) years on the same terms and conditions as the current lease (as
      amended) except that the rent for the option term shall be 95% of Fair
      Market Rent for similarly sized and outfitted premises in the Columbia,
      Maryland area rented in "as is" condition. Landlord and Tenant shall use
      commercially reasonable efforts to agree on the 95% of Fair Market rent by
      January 31, 2001. If no agreement is reached by January 31, 2001, Landlord
      and Tenant shall each appoint either a broker or an appraiser to represent
      their interest and the two parties chosen shall choose a broker or
      appraiser and each representative shall render its opinion on rent and
      escalations. If the three cannot agree on a compromise value, then the
      middle opinion shall be used.

         All the other terms of the existing Lease between Landlord and Tenant
shall remain unchanged.

         In witness whereof, the parties have executed this Second Amendment To
Lease Agreement as of the dates indicated below, the effective date of the
Agreement of Lease Amendment being the date of the final execution hereof by
both parties.

WITNESS:                                        LANDLORD:
                                                9033 Red Branch Road L.L.C.

/s/ Howard Swedley
-------------------                             /s/ S. Bruce Jaffey
DATE 6-15-00                                    ----------------------------
    --------                                    By: S. Bruce Jaffey
                                                Its: Managing Member

ATTEST:                                         TENANT:

/s/ Thomas A. Donelan                           Hemagen Diagnostics, Inc.
---------------------                           a Delaware corporation

DATE: 6-9-00                                    /s/ Jerry L. Ruyan
     -------                                    ----------------------------
                                                By: Jerry L. Ruyan
                                                Its: C.E.O.<PAGE>   1
                                 Exhibit 10.31
                              CONSULTING AGREEMENT

Upon the terms and conditions set forth below, the parties hereto agree as
follows:

1.    APPOINTMENT. Hemagen Diagnostics, Inc. (the "Company") hereby retains
      Thomas A. Donelan and Christopher P. Hendy (the "Consultants") to provide
      advisory services to the Company with respect to general corporate or
      strategic matters.
2.    FEES. In consideration for the services rendered by the Consultants, the
      Company agrees to pay the Consultants the following fees and other
      compensation:

         a)    The Consultants shall be paid at a rate of $60.00 per hour each.
               The fee shall be paid in cash for 25% of the amount earned and in
               Hemagen Diagnostics, Inc. common stock for 75% of the amount
               earned by each consultant. The price per share will be based on
               the average closing price for the last five trading days of each
               month in which services are provided.
3.    EXPENSES. The Company shall reimburse the Consultants for all
      out-of-pocket expenses incurred in rendering the services hereunder upon
      presentation by the Consultants of a statement of such expenses.
4.    TERM OF RETENTION. This Agreement shall terminate at the close of business
      on September 30, 2000 unless it is extended by agreement of the parties
      hereto.
5     MISCELLANEOUS. This Agreement shall be governed in accordance with the
      laws of the State of Ohio.

This Agreement constitutes the entire understanding and agreement between the
parties hereto with respect to the subject matter hereof and there are no
agreements or understandings with respect hereto which are not contained in this
Agreement.

Agreed and Accepted this 1st day of October 1999.

"Consultants"                                           "Company"

/s/ Christopher P. Hendy                              /s/ Jerry L. Ruyan
------------------------                              ------------------------
Christopher P. Hendy                                  Jerry L. Ruyan, CEO

/s/ Thomas A. Donelan
------------------------
Thomas A. Donelan

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