After a yearlong investigation, the Harvard Medical School has decided not to censure psychiatrist John Mack, a tenured professor and nationally known UFO researcher who has publicly asserted that extraterrestrial beings have "invaded our physical reality and are affecting the lives of hundreds of thousands, if not millions of people."

In a meeting held last Friday, Medical School dean Daniel Tosteson informed Mack that the review carried out by a "Special Faculty Committee" had ended, according to committee chairman Arnold Relman. "John Mack is just as he started out - a perfectly full-fledged member of the Harvard faculty with no adverse action taken," said Anne Taylor, a Harvard University counsel who sat on the committee convened by Tosteson last summer.

Mack had become a cause celebre at Harvard after the 1994 publication of his best-selling book, "Abduction." In countless television and newspaper interviews, he was inevitably dubbed "the Harvard professor who believes in UFOs," causing considerable anguish to many of his colleagues.

The Medical School committee's preliminary report, drafted earlier this year, chastised Mack for "affirming the delusions" of his many patients who claim to have been abducted by aliens. The committee also found Mack to be "in violation of the standards of conduct expected of a member of the faculty of Harvard University."

Mack's lawyer, Eric MacLeish, vigorously contested the preliminary findings and helped mobilize prominent academics to support Mack. The preliminary conclusions were toned down in the final draft sent to Tosteson a few weeks ago. It is unknown what restrictions, if any, will be applied to Mack's future work.

Neither Mack nor MacLeish was available for comment. In the past, Mack has said he hoped the conflict with Harvard could be resolved quietly and amicably. Now, "he's very relieved," a friend reported. "He feels he and the dean had a useful exchange."

The committee primarily concerned itself with the procedural details of Mack's work, according to two doctors who worked on the effort. "One of the issues before the committee was whether John was doing clinical care, or research," one doctor said. "If he was doing research on human subjects, he needed formal permission. He said it was clinical work." Committee chairman Relman said the board examined Mack's "professional behavior as an academic psychiatrist, and whether he was he dealing with his patients in a suitable academic way."

Contrary to published reports, Relman said the committee never considered revoking Mack's tenure at Harvard. "Harvard University isn't going to take action against someone who takes unorthodox views, and with whom it may disagree. John Mack may win the Nobel Prize and go down in history as the modern Galileo."

Mack, who has taught at Harvard for more than three decades, was a controversial figure even before he began his UFO research five years ago. Alternately described as a restless crusader or an intellectual butterfly, he has plunged himself into the anti-nuclear movement, mediation efforts in the Mideast and US-Soviet diplomacy, as well as a variety of alternative therapies, including "holotropic breathwork" and est. Mack served on the est board and introduced founder Werner Erhard to Harvard, "which went over like a lead balloon," according to one colleague.

Much of Mack's UFO investigations take place at his Cambridge-based Program For Extraordinary Experience Research. Many of his colleagues first learned of his UFO interests when he co-sponsored a national conference on alien abduction held at the Massachusetts Institute of Technology in 1992. While most of his colleagues privately scoff at the abductees' tales, they do credit Mack with many important contributions to psychoanalysis during his 35-year career.

Furthermore, many viewed the Medical School's review committee as an unacceptable intrusion into Mack's research. "I would defend to the death John's right to indulge in kooky stuff, in the name of academic freedom," said one psychiatrist.

The committee, whose proceedings were kept secret, met 25 times between summer 1994 and late spring of this year. Mack appeared several times before the group, as did a handful of his patients and several Harvard psychiatrists, who criticized his abductee research.

In the early weeks of the committee's proceedings, Mack and his first lawyer, Carl Sapers of Hill and Barlow, cooperated with the review board, turning over interview tapes and urging patients to tell their stories to the Harvard panel. "Initially, their approach was, 'What can you do to help us understand this research,' " said Peter Faust, an abduction "experiencer" who testified before the committee. "Then it became clear that Harvard had another agenda."

Mack later changed lawyers, engaging first Daniel Sheehan of the far-left Christic Institute, and then MacLeish, who specializes in mental-health related litigation.

In an interview earlier this year, a wistful Mack noted that the Harvard investigation "has been expensive in all senses of the word." In documents unrelated to the Harvard inquiry, Mack disclosed that he has already paid $30,000 in legal fees, and expects to be billed for $100,000 more.

Although Mack derives considerable income from family trust funds, his Harvard salary is $12,000 a year, which is not unusual for a clinician with limited teaching duties.