Mr. John, as he was discreetly called, came off as sort of the superstar of FBI informers, despite having to make his big appearance with a pillow case over his head.

His work had broken up counterfeit, drug and gun-running rings, he told a Senate subcommittee hearing last summer. His FBI control agents echoed the accomplishments of the man, who is now relocated with a new identity.

Law enforcement needs people like himself, Mr. John said, especially in the organized crime field, because undercover FBI agents would stick out. "They wear wingtipped shoes and class rings," was the way he put it.

On the other hand, Gray Thomas Rowe, who infiltrated the Ku Klux Klan in the 1960s and was recently indicted for murder, is an informer the FBI would like to forget about.

That is unlikely, though, because a special Justice Department task force was formed recently and the FBI's handling of him. It seems certain to document the worst kind of abuse by informers.

The activities of Rowe and Mr. John thus illustrate the extremes in an issue that is a continuing subject for debate: the value, use and control of informers as essential investigative tools. They also expressed great concern about protecting the identities of informers.

Bell, for instance, defied a federal judge in Manhattan in June rather than turn over confidential files naming individuals paid to spy on the Socialists Workers Party. He was held in contempt of court. Attorneys for the Justice Department and the SWP are scheduled to argue an appeal of the unprecedented case Wednesday.

Webster has suggested in recent speeches that there should be a 10-year investigative files because he fears prisoners and organized crime figures are using documents received under the Freedom of Information Act to try to identify informants.

Sens. Edward M. Kennedy (D-Mass.) and James Abourezk (D.S.D), who is retiring, have been interested observers of these events.

They are members of a Senate Judiciary subcommittee now drafting a proposed charter to spell out the duties of the FBI. They requested the Justice investigation of Rowe and his FBI handlers, and have heard witnesses, including Mr. John, in considering whether to put restrictions on the use of informers in the charter.

Bell and Webster have said they would rather not have investigative techniques limited by legislation. Internal regulations now in use are adequate, they say.

James Q. Wilson, a Harvard professor who has studied and written about informers, agreed with Bell and Webster during the Senate subcommittee hearings last summer. Wilson also noted that most of the reported abuses have involved informers who infiltrated domestic political groups, rather than those working in the criminal field.

The number of informers in both areas has dropped dramatically over the past few years. FBI figures show that at midyear there were about 2,800 informers in the criminal field and less than 50 in the domestic security area, most of those in identified terrorists groups. This compares with 10,000 informers on criminal activities and nearly 600 in domestic security in 1976.

The American Civil Liberties Union has suggested that Congress enact a warrant procedure to limit the use of informers to investigating criminal acts. There would be no warrants allowed for infiltrating political groups unless members were suspected of specific criminal conduct.

Philip Heymans, head of the Justice Department's Criminal Division, told the Senate subcommittee he recognized the "chilling effect" informers can have on domestic organizations. But he said a warrant procedure would be too limiting.

Other dangers, Heymann said, are that informers can provide unreliable information and that they may be required, at times, to take part in criminal activity to maintain their cover.

Departmaent guidelines say informers shall not participate in violent acts or initiate criminal activities.

The well-chronicled exploits of Garry Thomas Rowe demonstrate the need for such restrictions, in the opinion of many. Rowe has admitted killing a black man in Birmingham in 1963, and has been named as a prime suspect in a 1963 bombing of a Birmingham Baptist church in which four girls were killed.

In September, he was indicted by a county grand jury in Alabama for the 1965 slaying of civil rights worker Viola Liuzzo. Rowe had been the star prosecution witness at the trial of two former Klux Klan members charged in the case. They now say Rowe killed Liuzzo.

Any federal prosecution of Rowe is barred by the statue of limitations. The Justice task force headed by Ralph Hornblower III, therefore, is expected to concentrate on what FBI agents and Justice Department prosecutors knew at the time about Rowe's violent activities.

Even the FBI handlers of Mr. John the star FBI informer, were tempted at times to shade regulations. Anthony Villano, the control agent, told the Senate hearing that he once wanted his prize informer to make an illegal break-in during an investigation, but was overruled by a superior.

A Kennedy aide said the subcommittee is concerned about informers and their handling that makes that section of the proposed FBI charter one of the trickiest to resolve.