"Yeah, I fell down," said actor William L. Peterson of his recent accident during a performance of "In the Belly of the Beast" at the Kennedy Center. "But we've taken some of that physical stuff out of the show now."

Last night's performance at the Kennedy Center's Free Theater served to kick off a reception sponsored by the John Howard Association, an Illinois-based group that advocates reform for prisoners' conditions. In the gathering of about 30 were former Illinois senator Charles Percy and Rep. John Conyers (D-Mich.).

"I'd be more interested to extend this play if there was a way to elicit some sort of prison reform," said Peterson. Since acting in the play, Peterson has joined the board of directors of St. Leonard's, a halfway house in Chicago, and has discussed the possibility of working with the John Howard Association.

In "Beast," Peterson gives an intensely physical performance as convicted killer Jack Henry Abbott. He dislocated a shoulder during a recent performance, and his injury, coupled with a viral infection, closed the show for a few days last week.

"By dint of this play, Bill has become a real symbol for prison reform," said William Rentschler, the association's president.

Walking out of the theater after Peterson's graphic portrayal of the horrors of prison life, Percy leaned over to a friend and said, "I told my wife, 'You'd better not double-park or anything like that.' "

At the reception he was more serious.

"I introduced a bill called 'Work Your Way Out of Prison,' " he said, "with the idea being prisoners who are on good behavior and who would be getting out would get a chance to work their way out of prison."

"You walk down the street," said Peterson between receiving congratulations. "And you have no idea how many people you walk by, in the course of a day, who've either been to prison or are going to prison or should be and aren't."

The purpose of last night's gathering, said Michael Mahoney, the association's executive director, was to muster support for efforts to open up the Marion prison in Illinois to public monitors. Marion, one of the nation's federal maximum security penitentiaries, is "the only one in Illinois that is closed to monitoring," he said.

"Everybody's being denied access to Marion," said Rentschler. The prison (where Abbott is currently serving time) has been closed to outside groups since 1983, following prisoner violence, he said. He announced to the gathering that Conyers would be assisting in the association's efforts.

Is a play about Jack Henry Abbott, who killed a restaurant employe shortly after being released, the best way to encourage people to advocate prison reform?

"Yes," declared Peterson. "The system made [Abbott] that way. He was 4 years old when he went in. He didn't know anything else but the brutality of his prison experience. The play cries out for people to get involved with an institution that is huge in this country, that is not working.