EXHIBIT 5


                [KEATING, MUETHING & KLEKAMP, P.L.L. LETTERHEAD]


ROBERT E. COLETTI
DIRECT DIAL:  (513) 579-6559
FACSIMILE:  (513) 579-6956
E-MAIL:   RCOLETTI@KMKLAWCOM

                                 March 22, 1999


Ladies and Gentlemen:

     This firm is general counsel to Meridian Diagnostics,  Inc. and as such, we
are familiar with the Company's  Articles of Incorporation,  Code of Regulations
and corporate proceedings  generally.  We have reviewed the corporate records as
to the  establishment of the Company's 1999 Directors' Stock Option Plan and the
1996 Stock  Option Plan Amended and  Restated  Effective  January 22, 1999 which
calls for the issuance of shares of Common Stock to employees of the Company and
its  subsidiaries  upon exercise of options  granted to them.  Based solely upon
such examination, we are of the opinion that:

     1. The Company is a duly organized and validly existing  corporation  under
the laws of the State of Ohio; and

     2. The Company has taken all  necessary and required  corporate  actions in
connection with the proposed issuance of 50,000 and 500,000 shares  respectively
of Common Stock pursuant to the 1999  Directors'  Stock Option Plan and the 1996
Stock  Option Plan  Amended and Restated  Effective  January 22,  1999,  and the
Common Stock, when issued and delivered,  will be validly issued, fully paid and
non-assessable  shares  of  Common  Stock of the  Company  free of any  claim of
pre-emptive rights.

     We  hereby  consent  to be  named  in the  Registration  Statement  and the
Prospectus  part thereof as the  attorneys who have passed upon legal matters in
connection with the issuance of the aforesaid  Common Stock and to the filing of
this opinion as an exhibit to the Registration Statement.

                                         Very truly yours,

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



                                         By:/s/ Robert E. Coletti
                                            --------------------------------
                                                Robert E. Coletti