DEFENSE LAWYERS in Pennsylvania are making the same arguments on behalf of clients as Mayor Barry's supporters have been putting forward here. Why, they protest, is the U.S. Attorney's Office going after mere drug users when there are so many more important dealers and kingpins who should be getting attention? And because most simple users never become subjects of a full grand jury probe, isn't it selective prosecution to go after just a few? The answer, valid in Mayor Barry's case as well as the one developing in Harrisburg, is that the special position of these alleged drug users makes it not only reasonable but imperative that investigations, and if necessary, prosecutions, go forward.

The federal grand jury in Pennsylvania has been probing cocaine use by prominent businessmen and lawyers, some of whom have held high government positions. While no indictments have yet been handed up, court records show that two of the people being investigated are Richard L. Guida, former director of criminal investigations in the Pennsylvania attorney general's office, and Henry G. Barr, no less, who for nine months in 1988 and 1989 was special assistant to Attorney General Dick Thornburgh, charged with overseeing criminal investigations across the country. If these two men were cocaine users while at the same time being charged with prosecuting traffickers, they will have been guilty of a terrible betrayal of public trust and private duty. And if this is the case, it will be urgent to look into the possibility of corruption, blackmail and the subversion of investigations during their tenure.

Mr. Barr's relationship to the attorney general goes back many years. In the mid-'70s he served as an assistant U.S. Attorney in Mr. Thornburgh's office in Pittsburgh. Later, he was counsel to Gov. Thornburgh. Yet neither the attorney general nor any of his Pennsylvania associates at the Justice Department say they knew of his alleged drug use, and all have recused themselves from the case. Mr. Barr denied recent drug use on his security forms, and a full-field investigation by the FBI did not uncover any. Was the security investigation a thorough one? Was Mr. Barr using drugs while he held his Justice Department position, and if so, what is being done now to review all his work at the department for evidence of problems? Sen. Arlen Specter (R-Pa.) has asked for the appointment of an independent counsel, but if the Harrisburg grand jury is doing a thorough job, that may not be necessary. What is needed, though, is a full explanation to reassure the public that Mr. Barr did not compromise his office. Citizens of Washington are expecting full disclosure of Mayor Barry's actions with regard to city contracts, police decisions and personnel policies that might have been influenced by his alleged drug use. The same kind of report is obligatory in Mr. Barr's case.