U.S. Attorney General Griffin B. Bell won few converts but some respect yesterday after explaining to field agents his reasons for the prosecution of three former top FBI officials.

About 40 agents and other FBI employes listened quietly as Bell made an impromptu speech lasting nearly an hour from the center of the large agent's workroom at the Indianapolis FBI field office. He made the visit to talk with the agents following a keynot luncheon speech at the Indiana Bar Association convention here.

Moments before addressing the agents, Bell was handed a letter which began, "We would like to inform you that special agents of the Indiana polis FBI had planned to mass in front of the Indianapolis Convention Center during the time of your appearance at this location to demonstrate our support of the former agents who were recently indicted, and most importantly, our support of the FBI."

The letter, signed "Special Agents of the Indianapolis FBI office," said the demonstration had been canceled out of respect for the FBI, but added, "We are of the opinion in this field office that the FBI is being systematically destroyed for reasons unknown to us."

Bell, at the outset of his off-the-cuff remarks to the agents and employes, said, "I thank you for not demonstrating in front of the convention center."

He told the audience he was not particularly upset about the demonstration by hundreds of FBI agents Thursday in Washington outside of the courthouse when L. Patrick Gray, former acting FBI chief, Mark Felt, former associate director, Edward S. Miller, former intelligence chief, entered innocent pleas to charges they ordered illegal break-ins while pursuing Weather Underground terrorists. Bell added, however, "I would be worried if there were constant demonstrations, because someone has to run the FBI."

Bell spent about 20 minutes explaining his position on prosecuting the three men and then responded to written questions from the agents.

The first question asked was what he expected the prosecutions to accomplish.

"The same as any other prosecution," Bell said, "to uphold the law."

Asked if the purpose of the prosecutions was more to serve as a warning to present agents rather than to punish former agents, Bell replied, "I guess someone could say that, but that's not the basis of my decision."

After responding to several more written questions, Bell asked the agents if they had any oral questions. An agent asked if Bell thought the majority of people supported his position.

Bell said he thought that most people had not made up their minds on the matter, adding, "By having the virgil you had yesterday you will probably stir up some sympathy for these men."

Bell said he realized the alleged acts happened during unusual times. "The prosecution must go forward," he said, "but we wish they would not have much done to them in the end."

The only audible reaction Bell received during his appearance was when he was asked if he had thought about not prosecuting Gray, Felt, and Miller.

There was a slight ripple of laughter when he replied, "Yeah, I thought about it a lot. It would have been the easiest thing not to prosecute. I can take the flak from the left better than from the right, and you know I've got to return to Georgia some day to face all those conservatives."

Near the end of his presentation an agent asked how he and his colleagues could help the FBI. Bell told them to do their job, work on getting a congressional charter for the FBI, and said, "get rid of this hang-dog attitude that somebody's after you. Nobody's after you. I'm not after you."