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                                                                       Exhibit 5
   
                                 June 28, 1996
    

National Diagnostics, Inc.
737B West Brandon Blvd.
Brandon, FL  33511

         RE:     Registration Statement on Form SB-2

Ladies and Gentlemen:

   
         This opinion is being furnished in connection with the Registration
Statement on Form SB-2 (the "Registration Statement"), of National Diagnostics,
Inc. (the "Company"), under the Securities Act of 1933, as amended (the "Act"),
for the registration of 742,864 shares of common stock, no par value consisting
of 640,159 shares (the "Selling Shareholder Shares") being offered by the
selling shareholders named in the Registration Statement and 92,705 shares (the
"Warrant Shares") issuable upon exercise of outstanding warrants (the
"Warrants") issued pursuant to a Warrant Agreement dated as of September 27,
1994 by and between the Company and American Securities Transfer, Inc.

         As counsel for the Company, we have examined and are familiar with (i)
the Articles of Incorporation and Bylaws of the Company; (ii) the proceedings
of the Board of Directors of the Company relating to the issuance of the
Selling Shareholder Shares and the Warrants; and (iii) such other Company
records, documents and matters of law as we have deemed to be pertinent.
    

         Based upon our examination of such documents and our familiarity with
such proceedings, it is our opinion that:

         1.      The Company has been duly incorporated and is validly existing
and in good standing under the laws of the state of Florida.

   
         2.      The Selling Shareholder Shares are duly authorized, validly
issued, fully paid and non-assessable.

         3.      The Warrant Shares are duly authorized and will, when issued
pursuant to the due exercise thereof against payment of the exercise price
thereof, be validly issued, fully paid and non-assessable.
    

         We hereby consent to the inclusion of this opinion as Exhibit 5 in the
Registration Statement and to the reference to this firm under the caption
"Legal Matters" in the prospectus.  In giving this consent, we do not thereby
admit that we come within the category of persons whose consent is required
under Section 7 of the Act or the rules or regulations of the Securities and
Exchange Commission promulgated thereunder.

                                     FOLEY & LARDNER



                                     By: /s/ Linda Y. Kelso
                                        -----------------------------------
                                           Linda Y. Kelso