IN PREPARING to exhibit the controversial traveling exhibit of photographs by Robert Mapplethorpe last April, officials of the Contemporary Arts Center of Cincinnati tried to be prudent: they restricted access to the museum to those over 18 and relegated the most disturbing and explicit of the 175 photographs to a separate, further isolated room. This was not enough, however, to keep a local sheriff from staging a raid on the exhibit and seeking indictments against museum director Dennis Barrie on obscenity charges; nor was it enough to keep the grand jury from handing up an indictment.

Judge David J. Albanese of the Hamilton County court ruled earlier this month that the charges had to go to trial and that the five designated "obscene" photographs cannot even be considered in the context of the entire Mapplethorpe show. The five pictures -- two portraits of partially nude children, plus several depicting sadomasochistic activities -- must be tried as separate exhibits. The judge's ruling, which controverted much previous law and portions of the Supreme Court's obscenity definition, does not necessarily spell a conviction. But it limits the kinds of evidence that can be introduced in defense of the exhibit and the museum. The judge declared that, far from being an integral part of the exhibit, the photos were in fact something for which the rest of the exhibition was "a vehicle" -- this despite the exhibition's billing as a retrospective of all aspects of the late photographer's career, and despite the reputation of Mapplethorpe and the favorable reactions of many to his work on artistic grounds.

Jury selection continues in Cincinnati for the trial of a case that should never have been brought. This, remember, is no matter of federal money, but specifically of the right of American adults to view material that other adults consider unacceptable. Mr. Barrie faces fines, heavy legal fees and a possible jail term, for hanging pictures in an art museum. If the First Amendment, which protects unpopular expression, doesn't apply to this kind of situation, it's hard to imagine where it does.