Judith Campbell Exner, who once had concurrent liaisons with President John F. Kennedy and Chicago Mafia boss Sam Giancana, said in an interview published yesterday that she served as a courier between Kennedy and the Mob, at Kennedy's request.

In an interview published in People magazine, Exner, 54, said she lied 13 years ago in first describing her affair with Kennedy, when she said the assassinated president was unaware of her friendships with mobsters.

"He knew everything about my dealings with Sam Giancana and Johnny Roselli because I was seeing them for him," Exner told author Kitty Kelley. "I wouldn't have been seeing them otherwise. And I would never have known mobsters if it hadn't been for Frank Sinatra."

Exner, who was paid $50,000 for the interview, said she was speaking up now because she has terminal cancer and has been told by her doctor she has only about three years to live. "I lied before to protect myself," she said. "If I'd told the truth, I'd have been killed. I kept my secret out of fear."

Exner said she arranged about 10 meetings between Kennedy and Giancana, including one she said she believes took place at the White House on Aug. 8, 1961. She said Giancana showed up at her Washington hotel room that day and told her he had just come from a meeting with Kennedy.

"There were many more meetings, but I don't have all of my paper work and I don't want to take a chance" and go further, she said in an appearance yesterday on ABC-TV's "Good Morning America" show.

Exner's involvement with Kennedy first came to light in 1975 during the investigation by the Senate Select Committee on Intelligence of CIA plots to kill Cuban leader Fidel Castro with the help of Giancana, his West Coast lieutenant Roselli, and Florida Mafia boss Santo Trafficante.

Giancana was shot in the head in the kitchen of his Oak Park, Ill., home on June 19, 1975, as Senate investigators were preparing to contact him. The corpse of Roselli, who did testify before the committee about the CIA-Mafia plotting, was found in an oil drum weighted down with heavy chains in 1976 in a bay near Miami.

Exner's latest account raised immediate skepticism in some quarters, especially in light of heavy FBI surveillance of Giancana in the early 1960s and constant Secret Service details surrounding the president.

"I find it incredible that John Kennedy would have met Sam Giancana face-to-face after he was president," said G. Robert Blakey, a University of Notre Dame law professor who directed a House investigation of the JFK assassination in 1978 and who believes the Mafia killed Kennedy. "The two principals would never have met."

Exner said Sinatra introduced her to Kennedy in Las Vegas on Feb. 7, 1960, and their affair began the next month at the Plaza Hotel in New York on the night before the New Hampshire primary. She said Sinatra introduced her a week later in Miami Beach to Giancana, calling him "Sam Flood."

According to Exner, Kennedy asked her at a dinner meeting in his Georgetown town house on April 6, 1960, to arrange a meeting with "Sam" in view of the forthcoming West Virginia primary. "I think I may need his help in the campaign," Exner quoted Kennedy as saying when she asked him why.

She said she flew to Chicago, met Giancana and arranged an April 12, 1960, meeting at the Fontainebleau hotel in Miami Beach. Exner said Kennedy came up to her suite afterward and when she asked him how it had gone, "he seemed very happy about it." She claimed Kennedy also gave her an envelope with two $1,000 bills in it.

Exner said Kennedy asked her to arrange several more meetings after he won the nomination. "After Jack was elected," she said, "Sam kept saying he would never have been president if it hadn't been for his efforts on Kennedy's behalf in Illinois."

Bill Roemer, a former FBI agent assigned to Giancana, said the Mob did control the First Ward Democratic organization and Giancana often boasted he had elected Kennedy, but he said the late Chicago Mayor Richard J. Daley "had a lot more influence with influential people in Chicago than Giancana did."

Roemer was also skeptical of Exner's report of a Kennedy-Giancana meeting in her suite at the Ambassador East Hotel in Chicago on April 28, 1961, shortly after the Bay of Pigs debacle.

"That would be something we likely would have picked up," Roemer said of the electronic surveillance the FBI was conducting at the time of Mob headquarters in a second-floor tailor's shop on Michigan Avenue. "What we had there was a daily meeting of about half a dozen of the top leaders of the Chicago Mob."