FBI Director William H. Webster told a House subcommittee yesterday that the members of Congress implicated in the Abscam undercover bribery investigation had fair warning they were taking part in criminal activity.

Webster made the remarks in response to a sharp attack on the FBI's use of allegedly corrupt middlemen to approach members of Congress with bribes.

Several members of the House Judiciary subcommittee on civil and constitutional rights criticized the technique whereby FBI undercover operatives allowed the middlemen to bring public officials before secret videotape cameras without real evidence the member was corrupt.

Webster cautioned that he did not want to discuss the pending Abscam investigation, which press reports have said implicates seven House members and one senator in trading promises of legislative action for cash payments. But he clearly was referring to the congressional scandal in several of his answers.

"They [the Abscam middlemen] were told consistently not to bring someone to the undercover operation unless that person was prepared to make promises up front and to take the money personally," Webster said.

He added that as an extra precaution, it was made clear to the individual members involved that the activity was criminal.

Webster said he was confident that any trails resulting from the investigation "will give validity and vindication to the processes we put in place."

Webster's strong defense of the government's techniques in Abscam were echoed generally by Philip B. Heymann, head of the Justice Department's Criminal Division.

At the end of the hearing, several members of the subcommittee, still bothered by the role of the middlemen, indicated they may press for restrictions on their use in the pending legislative charter for the FBI.

Rep. Peter W. Rodino Jr. (D-N.J.), chairman of the full committee, said he was concerned about the FBI's apparent dependence on uncontrollable intermediaries who claimed they had influence with public officials.

Though Rodino didn't refer to it in the hearing, one New Jersey middleman, Joseph Silvestri, tried to approach Rodino during the Abscam operation through the congressman's son-in-law, Charles A. Stanziale.

Stanziale has said he refused an offer of $50,000 just to set up a meeting with Rodino.Silvestri also approached several other members of Congress, most of whom refused meetings or refused to take the bait.

Sources close to the investigation said, for instance, that when Silvestri brought Sen. Larry Pressler (R-S.D.) to the northwest Washington house used in the operation, it was obvious that the senator had no idea why he was there. Thus the undercover agents did not press the overture and Pressler walked out.

Rep. John F. Seiberling (D-Ohio) noted that Pressler had gone to the meeting because he was told someone was interested in giving him a campaign contribution -- a legitimiate transaction.

Seiberling said the committee "owed it to the Congress and the American people to explore to what extent honest motives [of members] were used to suck people into some kind of trap." He said public officials shouldn't have to prove their honesty.

Heymann said during the hearing that he agreed the Justice Department wasn't "in the business of testing morality." But he also said he didn't think it was unfair "to expect an official to turn down what is plainly a bribe."

The FBI is seeking an increase in its undercover operations budget this year from $3 million to $4.8 million. Webster noted that 15 undercover operations had to shut down last year because of budgetary restraints.

Rep. Don Edwards (D-Calif.), chairman of the subcommittee, said after the hearing that Congress has gone along with increased funding for FBI undercover operations, but also has a duty to audit the performance of such techniques as the use of middlemen.

Webster took pains to point out that the middlemen in Abscam -- such as Silvestri and Philadelphia lawyer Howard Criden -- were themselves targets of the investigation.

He said the undercover operation started out seeking to recover stolen art objects. "Some people who were bringing us thieves began bringing us people of influence, people willing to sell their office," Webster said.

When challenged about why the FBI listened to the middlemen. Webster said it was necessary to reach beyond the street criminals usually caught in such operations. And he noted that the middlemen started proving their claims of influence when they produced the members as they said they could.

Heymann said he felt there was little harm in exposing innocent public officials to the Abscam middlemen. He said that the courts have been very liberal with what federal law enforcement officers can do in undercover operations but the department adds its own safeguards, including making it clear to targets that the actions being discussed are illegal.

He also said that undercover operations infringe less on civil liberties than wiretaps or searches or sweeping grand jury subpoenas because the targets choose to come into the net themselves.

Undercover operations also have clear advantages over other investigative techniques, he added.

The operations let law enforcement agents infiltrate secret organizations, they are completely accurate when videotaped, and they "have a spectacular deterrent effect."

In a related matter, both Webster and Heymann said they were among 100 FBI and other Justice Department officials questioned about the leaks that produced the Abscam stories. They, like others, are prepared to take lie detector tests to prove they didn't give out information to the four news organizations that broke the story a month ago.