THE DECISION of Judge Frank M. Johnson, Jr. that he cannot accept the nomination as director of the Federal Bureau of Investigation is, as Jody Powell said, "a loss to the administration, to the FBI and to the country." Judge Johnson would have given the FBI the kind of aggressive and thoughtful leadership it needs as it attempts to re-establish the great reputation it once had. But the judge cannot be faulted for concluding it would be unfair to keep his nomination hanging any longer during his unfortunately long period of convalescence.

His decision, of course, puts the administration and, most particularly, Attorney General Griffin Bell back on square one in the search for an FBI director. It is just as well that Mr. Bell has said he will "let the dust settle" for a couple of weeks before deciding on how to renew that search. The method he used last winter, when a selection committee considered more than 200 persons and sent five names to President Carter, was not notably successful. None of the five received much public or political support when their names become known, and the President went outside of the list to nominate Judge Johnson.

The search may be even harder now. By picking Judge Johnson, President Carter plainly identified the qualities he thinks are needed for the next director of the FBI. The public reaction to his choice suggests he is right. The result is that the President's next nominee will be judged by comparison with Judge Johnson, not by comparison with other potential nominees or prior directors. Finding someone who can meet that test will not be easy.