EXHIBIT 10.51




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                                FOURTH AMENDMENT

         This Fourth Amendment (the "4th Amendment") is made and entered into as
of this 27th day of December 1999 by and between NATIONAL DIAGNOSTICS, INC., an
Florida corporation ("NDI") and American Enterprise.com, Corp., formerly known
as American Enterprise Solutions, Inc., a Florida corporation ("AESI").


                                    RECITALS

         WHEREAS, NDI and AESI have entered into that certain Merger Agreement
dated February 23, 1998 as amended by that certain First Amendment dated March
17, 1998 and that certain Second Amendment dated April 29, 1998 and that Third
Amendment dated July 24, 1998 (the "Agreement") pursuant to which it is
contemplated with AESI will be merged (the "Merger") with and into NDI under the
terms and conditions specified in the Agreement; and

         WHEREAS, Section 4.1 of the Agreement currently provides, among other
things, that the Closing of the Merger shall occur on July 31, 1998, or as soon
as practicable after all conditions to Closing shall have been satisfied or
waived, or at such other time and date as NDI and AESI may mutually agree; and

         WHEREAS, AESI and NDI have mutually agreed that the date of Closing the
Merger shall be on or before December 31, 2000, or as soon as practicable after
all conditions to Closing shall have been satisfied or waived, or at such other
time and date as NDI and AESI may mutually agree; and

         WHEREAS, Section 13.1(ii) of the Agreement currently provides, among
other things, that AESI (acting through its board of directors) shall have the
right to terminate the Agreement if the Closing shall not have occurred by
December 31, 1998; and


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         WHEREAS, AESI and NDI have mutually agreed that Section 13.1(ii) of the
Agreement should be amended to reflect the fact that AESI (acting through its
board of directors) shall have the right to terminate the Agreement if the
Closing has not occurred by December 31, 2000.

         NOW, THEREFORE, in consideration of the premises set forth herein, and
for other good and valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, the parties hereto, intending to be legally bound,
agree that the Agreement is hereby amended to incorporate and reflect the
following facts, terms and conditions:

         Section 4.1 of the Agreement is hereby amended to read as follows:

                 4.1    PLACE AND DATE OF CLOSING. Delivery of the stock
                        certificates referred to in Section 3 above, and
                        consummation of the other transactions contemplated by
                        this Agreement (hereinafter referred to as the
                        "Closing") shall take place at the offices of AESI, 6800
                        N. Dale Mabry, Suite 100, Tampa, Florida 33614 (or at
                        such other location as may be agreed upon by AESI and
                        NDI) on or before December 31, 2000, or as soon as
                        practicable after all conditions to Closing shall have
                        been satisfied or waived, or at such other time and date
                        as NDI and AESI may mutually agree, which date shall be
                        referred to as the "Closing Date."

         Section 13.1(ii) of the Agreement is hereby amended to read as follows:

                 (ii)   by AESI (acting through its board of directors) if the
                        transactions contemplated by this Agreement to take
                        place at the Closing shall not have been consummated by
                        December 31, 2000, unless the failure of such
                        transaction s to be consummated is due to the willful
                        failure of such transactions to be consummated is due to
                        the willful failure of AESI to perform any of its or his
                        obligations under this Agreement to the extent required
                        to be performed by it prior to or on the Closing Date:
                        or

         This Amendment may be executed in two or more counterparts, each of
which shall be deemed an original and all of which together shall constitute but
one and the same instrument.

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         IN WITNESS WHEREOF, the parties hereto have executed this Amendment as
of the day and year first written above.

                                          NATIONAL DIAGNOSTICS, INC.

                                          BY: /s/ CURTIS L. ALLISTON
                                              --------------------------------
                                          Name:  Curtis L. Alliston
                                          Title: President & Director



                                          AMERICAN ENTERPRISE.COM, CORP.

                                          BY:  /s/ CARDWELL C. NUCKOLS, PH.D.
                                               -------------------------------
                                          Name:  Cardwell C. Nuckols, Ph.D.
                                          Title: President & Director