At least two of the undercover agents in the FBI's Abscam investigation posed not only as associates of a fictitious Arab sheik, but also as associates of a very real U.S. senator -- Edward M. Kennedy (D-Mass.) -- according to some persons familiar with the probe.

The two, according to the sources, were an agent who called himself "Jack McCloud" and who claimed to be a Kennedy family financial adviser, and a female agent who went by the name "Margo Kennedy" and who claimed to be a cousin of the senator and a friend of the sheik.

Critics of the operation argue that use of the Kennedy name was improper, because it made difficult for Democratic politicians to walk away without at least listening to what the undercover agents had to say. Justice Department officials and Kennedy aides say they were unaware of such a ploy, and one FBI official, asked for comment, said "McCloud" denied that anything like this ever took place.

The alleged use of Kennedy's name is only a minor element in the 16-month "sting" investigation, one of the largest ever conducted into high-level political corruption. But it illustrates several of the concerns about the undercover operation that have been voiced in recent weeks and that probably will be heard in congressional forums starting March 3, when a House Judiciary subcommittee begins hearings on the techniques used in Abscam.

The concerns include questions about what sorts of phony covers are proper, how targets of investigations are picked, whether entrapment -- or something very close to it -- is inherent in such operations, how much control the government has over undercover agents and informants, and the lengths to which the government can go to keep undercover investigations from being exposed.

Underscoring these and other issues is the question of how tightly such operations can ever be controlled. By nature, undercover operations require that operatives be given a long leash and be permitted to make spot decisions on their own. Often, that is done without consulting headquarters, as apparently was the case in "McCloud's" allegedly passing himself off as a financial adviser to the Kennedy family.

In that situation, according to persons close to some of the subjects of the investigation, the agent boasted of such ties last spring, both while chartering a plane to fly associates of Sen. Harrison Williams (D-N.Y.) to Virginia to inspect a titanium mine, and while at a party on a Florida yacht that the FBI was using to make the undercover operatives appear wealthy.

Philip Heymann, head of the Justice Department's criminal division, said in a recent interview that there are always some problems of control in undercover operations.

"It has to be remembered that the process is going to be taking place a long way from Washington -- out in the field," he said. "Reports come in late, well after the fact . . . and a lot of decisions have to be made on the spot."

But those decisions are mostly minor, Heymann said.