The Kennedy family has hired a street-smart drug expert from Sacramento, Calif., to live with 24-year-old David Kennedy in an unusual, around-the-clock, "One-on-one" therapy program that has had remarkable success with others in the past.

Donald F. Juhl, whose friends say he "looks like a cowboy and talks like a cowboy but is wise in the ways of the streets," has been living with the family of the late Sen. Robert F. Kennedy since November, a source close to Juhl said yesterday.

Juhl is on leave from the Aquarian Effort, a non-profit drug treatment center in Sacremento which was started in the late 1960s and has specialized in the problems of upper middle-class youths with drug problems.

David was hospitalized last September for treatment of a heart infection that is sometimes associated with narcotics addiction. He had been robbed of $30 a week earlier in a Harlem hotel known to be frequented by drug pushers and addicts.

Juhl, reached yesterday at the Kennedy Compound in Hyannis Port, said he was there with David, but referred all questions to David's uncle Steven Smith or David's mother, saying that his presence there was of a "confidential nature."

Juhl was recommended to the Kennedys by a top Democratic state official in California whose son had been helped by Juhl "when all else failed," the source said.

Smith hired Juhl after checking him out with medical experts on both the East Coast and the West Coast.

Juhl was asked by the Kennedys not to tell anyone with whom he would be working. But the identity of his patient leaked out when he had to explain his continuing absence to board members at Aquarian Effort.

Part of his technique is never to leave his patient alone. With an assistant, Juhl has been with David Kennedy at Hyannis Port and Hickory Hill and during a skiing vacation in Aspen, Col.

Juhl is reportedly so optimistic that he has told Aquarian Effort officials that he will be back March 1. If not, David is expected to move to California to continue with Juhl for as long as necessary.

Former CIA director Richard Helms says that it is not true that he recently called muckraker Jack Anderson an "a-----" at a party and threatened the columnist's assistant with a karate chop.

"I can't imagine I said that," Helms says with amusement in his voice. "I have other terms I use for Mr. Anderson . . . I'm sure I never said that word."

Helms says that he did have an "animated" exchange with Anderson's aide, Les Whitten, on the evening in question, "but nobody threatened anybody."

Whitten's version is that he and Helms got into a "heated" argument and that Helms drew his hand back menacingly at one point.

"I thought he was going to hit me," Whitten says. "I told him to go ahead and hit me . . . which was pretty foolish, since I'm sure he's a black belt."

Helms would neither confirm nor deny that he is a black belt.

The confrontation is the latest in a years-long feud between Anderson and Helms. On Jan. 2, Anderson wrote a column stating that the shah of Iran had been Helms' "biggest-spending client" when Helms opened a consulting firm here.

Helms wrote a letter to the editors of The Washington Post denying that he had ever received "a single cent or rial" from "the shah or interests controlled by him."