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                      KEATING, MUETHING & KLEKAMP, P.L.L.
                                ATTORNEYS AT LAW


                                September 23, 1996


Direct Dial: (513) 579-6560


Meridian Diagnostics, Inc.
3471 River Hills Drive
Cincinnati, Ohio 45244

Gentlemen:

     We have examined the corporate records and proceedings of Meridian
Diagnostics, Inc. (the "Corporation") with respect to: 

     1. The organization of the Corporation;

     2. The legal sufficiency of all corporate proceedings of the Corporation 
in connection with the creation and issuance of all of the present outstanding 
and issued Common Stock of the Corporation; and
  
     3. The legal sufficiency of all corporate proceedings of the Company taken 
in connection with the authorization of the issuance of $12,500,000 principal 
amount of Convertible Subordinated Debentures (plus up to an additional 
$1,875,000 principal amount to cover an over-allotment option) (the 
"Convertible Debentures") all to be issued in a public offering pursuant to a 
Registration Statement filed with the Securities and Exchange Commission on the 
date hereof.

     Based upon such examination, we are of the opinion:

     1. That the Corporation is a duly organized and validly existing 
Corporation under the laws of the State of Ohio and has the power to borrow 
money, to issue the Convertible Debentures and to execute the Indenture 
pursuant to which the Convertible Debentures will be issued (the "Indenture");

     2. That the Corporation has taken all necessary and required corporate 
actions in connection with the proposed issuance of the Convertible Debentures 
and that when, and if, issued and delivered, the Convertible Debentures will be 
validly authorized, legally issued and binding
   

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Meridian Diagnostics, Inc.
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August 29, 1996


obligations of the Corporation enforceable according to the terms of the 
Convertible Debentures and the Indenture; and

        3. That all shares of Common Stock of the Corporation issuable upon
conversion (pursuant to the terms of the Indenture) have been duly reserved for
issuance upon such conversion and that when, and if, issued upon conversion of
the Convertible Debentures, will be validly authorized, duly issued and
non-assessable shares of Common Stock free of any pre-emptive rights.

        We hereby consent to be named in the Registration Statement and the 
Prospectus part thereof as the attorneys who will pass upon legal matters in 
connection with the issuance of the Convertible Debentures and Common Stock of 
the Corporation registered in connection therewith and to the filing of this 
opinion as an exhibit to the Registration Statement, and furthermore consent to 
the references made to this firm in the Registration Statement.

                                        

                                        Yours truly,

                                        KEATING, MUETHING & KLEKAMP, P.L.L.
                                        

                                        BY: /s/ ROBERT E. COLETTI
                                           -----------------------------------
                                                Robert E. Coletti