A federal grand jury is investigating allegations of conspiracy to obstruct justice involving a witness in the bribery conspiracy trials of Dr. Murdock Head, founder of the Airlie Foundation convicted of plotting to bribe members of Congress, according to sources familiar with the inquiry.

The investigation focuses on whether Airlie officials conspired with a Virginia prosecutor to indict a former foundation employe who testified against Head and acknowledged embezzling funds from Airlie in the process.

Roger Inger, a deputy commonwealth's attorney in Fauquier County, said in an interview that several months after Head's 1979 conviction, Frank Kavanaugh, acting foundation director, urged prosecution of Robert Curtis, who had acknowledged taking $25,000 from Airlie in the early 1970s. According to Inger, Kavanaugh told the county prosecutor: "They got the Doc and now we want Curtis."

Kavanaugh this week denied making the statement. "No, no," Kavanaugh said. "I've never felt they've gotten 'Doc.' "

Kavanaugh, who acknowledged he met with the state prosecutors, said that federal lawyers in Alexandria had questioned whether the Curtis investigation was intended to "chill" any future testimony by Curtis against Head, an allegation Kavanaugh denied. Under federal law, it is illegal to interfere with or attempt to influence the testimony of any witness in a criminal proceeding.

"They want to know whether there was a conspiracy to go after anyone who fell from grace in Murdock Head's world," said Kavanaugh. "It's absolutely untrue."

The allegations are the latest in a five-year federal investigation of Head, a George Washington University professor twice found guilty of conspiring to bribe two congressmen in the mid-1970s in an alleged attempt to win lucrative government contracts for Airlie and its affiliates. Head founded Airlie, a conference center and group of affiliate organizations, near Warrenton, Va., in the 1960s.

Head, who was retried and convicted again after his first conviction was overturned, has declined to comment on the current investigation. "All I can say is, if I felt any better I'd have to buy more insurance," he joked in a telephone interview. Head, who is on paid leave from GW, is free on bond pending an appeal of his conviction and a 4 1/2-year prison sentence he was given last July.

Federal prosecutors previously attacked the propriety of the local investigation, charging that Fauquier Commonwealth's Attorney Charles Foley and Inger, his deputy, had a conflict of interest because they had Airlie and some of its employes as clients in a part-time private law practice during the time of the Curtis investigation. The local prosecutors maintained there was no impropriety, but later stepped aside from the case.

A Virginia judge subsequently appointed a special prosecutor to continue the Curtis investigation, which led to Curtis' indictment in September 1980 and a later guilty plea by him. Curtis appeared again as a government witness in Head's retrial last summer in Alexandria. "He wasn't a great damaging witness or anything," said Kavanaugh. "A nice man, though."

Kavanaugh, who said he has furnished subpoenaed records to the grand jury but has declined to answer its questions under the Fifth Amendment, said Airlie officials knew of the missing funds before Curtis' testimony, but believed he had taken perhaps $1,400 to $1,800. Curtis was fired in 1975, shortly after the discovery, Kavanaugh said.

He said he was stunned by Curtis' statement during Head's 1979 trial that the embezzlement totaled $25,000. Kavanaugh said it was then that he went to Fauquier prosecutor Foley and asked him to pursue the matter "to seek restitution."

The current federal investigation also has examined a $54,000 consulting fee allegedly paid to Head by a film-making branch of the Airlie organization following his initial conviction in October 1979, the sources said. A lawyer involved in the case said federal prosecutors are investigating a possible contradiction between Head's testimony that he served the film affiliate as an unpaid consultant and his receipt of the fee allegedly for past services.

Prosecutors charged at the Head trials that Head paid about $49,000 in bribes during the 1970s to former representatives Daniel Flood and Otto Passman out of a slush fund that Head allegedly amassed through phony expense vouchers and double billings on film projects. Head contended the money in question came from personal loans totaling some $50,000 from the Raven's Hollow film affiliate and denied it was used to buy congressional influence.

William B. Cummings, a lawyer who has represented another Airlie employe called before the grand jury, said the panel has questioned Head's repayment of the Raven's Hollow promissory notes, which occurred about the same time Head allegedly received the consultant's fee shortly after his initial conviction.

Head said he has had little trouble keeping busy at Airlie while he awaits the outcome of his appeal, scheduled for argument in early July. He is chairman of the executive committee of the Fauquier County Rescue Squad, he said, and recently completed a film on voluntarism, entitled "Together," for use by rescue squads around the country.

Head also played a key role in the making of "The Shooting of the President," a 30-minute television special about the attempted assassination of Ronald Reagan that was broadcast recently on Channel 7 in Washington and on ABC-TV. However, the executive producer's credit line went to Airlie director Kavanaugh, because, Head said, "I didn't want my name to cast a cloud on it."