James G. (Bo) Gritz was hardly your run-of-the-mine Capitol Hill witness. He is a lean, swaggering man of action, and his talk was a flow of riveting stuff about clandestine operations, jungle raids, intrigues and river crossings.

The former Green Beret commander even took the oath with a great flourish. "So help me God, I do," he said with a sweep of his arm, his blue eyes gleaming in the lights of dozens of TV and still cameras.

His narrative was a compelling change from the standard stuff about cost overruns and budget deficits. After Gritz delivered 45 minutes on "Operation Velvet Hammer," jealous bureaucrats, of a perfidious "brother" who blew the cover on the "Operation Lazarus" in mid-course, and gave a breezy tour of the Oriental mind, Rep. Stephen J. Solarz (D-N.Y.), chairman of the House Asian and Pacific affairs subcommittee, politely sought to intervene.

"I see you are bored," Gritz said jauntily.

Solarz demurred. "I have never heard any testimony quite like this," he said. "It is fascinating."

The fascination of his story is beyond dispute. The question is whether Gritz believes it, specifically whether he believes that there are still Americans held against their will in Laos and Vietnam. He blasted the latest Vietnam controversy into the headlines last month by getting caught in Thailand, where he had led a private army on an unauthorized rescue mission.

Gritz admits he cannot offer proof positive that there are American captives in Southeast Asia. The photographs he had taken by a Laotian agent didn't come out. The camera was set wrong, and the agent apparently shot the scenery.

"I have the same evidence that might be presented by a clergyman to convince you that God exists," he said cheerily, as members tried to pry something more concrete out of him.

Gritz is not the only one who says there are U.S. servicemen being held against their will, 10 years after the last American came home from the war.

Some members of the National League of Families, who were in the jammed hearing room, agree with him, more out of hope and faith than any evidence. Their executive secretary, Anne Mills Griffiths, followed Gritz to the stand to say his derring-do had "seriously interfered with the accounting of the missing, including the recovery of remains."

At one point, at least, high officers of U.S. intelligence services gave credence to "live sightings" of Americans reported by the "boat people" who left Vietnam in 1975 and were encouraged, in their refugee camps, to come forward with any information they might have about having seen Caucasian men. A general in intelligence, since deceased, induced Gritz to leave the Army to undertake the POW search.

Now it is different.

"The government says I am spooking their own operation," Gritz said, and went on to note that the sum of their efforts is zero--which is of course what his is, too.

Among those listening to the swaggering colonel was Bobby Muller, the sardonic paraplegic who is president of the Vietnam Veterans of America. He says he thinks Gritz is one of those clandestine types who can never find a life other than leading a column through the jungle with a gun on his hip and a walkie-talkie in hand.

"This is ridiculous," he said when Gritz had finished. "He is exploiting people. This issue should be put to rest. How could anyone survive in a jungle camp for 10 years?"

Those views are devoutly echoed by Rep. G.V. (Sonny) Montgomery (D-Miss.), who was chairman of a now-defunct select committee on missing servicemen, which reported, after a year's study and several trips to Southeast Asia, that there are no Americans left alive there.

Montgomery says he thinks Gritz is "sincere" in his belief, but does wrong to raise the hopes of the families. He says he regrets that the president gave the poisoned pot another stir when he went before the National League of Families in January, gave the problem of the missing-in-action "highest national priority" and mentioned the "live sighting" reports.

Gritz denies soliciting funds for his operation, although he admits he raised between $150,000 and $200,000. There are reports that he raised $10,000 on a book he is writing and accepted money from POW families.

Perhaps Gritz' most bizarre offering was his explanation of why the Vietnamese and Laotians would hold the Americans for so long in secret: Hanoi wants to use them as a bargaining chip, to extort from the United States the $3 billion in reparations Richard M. Nixon promised the North Vietnamese and never delivered.

How they can use them for blackmail without acknowledging their existence is not clear.

And neither was much else at the hearing. The larger question is not whether Gritz is a crusader or a hustler, but whether the last bitter chapter of that bitter war will ever be written.