SAN FRANCISCO -- For many of those who attended, the Sixth International Conference on AIDS ended here on Sunday with disappointment and regret.

Few important scientific advances were reported, but most researchers hadn't expected any. The streets, blocked in creative new ways each day by vigilant demonstrators, were difficult to navigate. But even that did not distress most delegates.

Yet when protesters from ACT UP -- the Aids Coalition to Unleash Power -- prevented Health and Human Services Secretary Louis W. Sullivan from delivering a closing address that delegates could hear, many people felt the demonstrations had gone too far.

"It's just you get this sense of futility sometimes," said Paul Volberding, the director of the AIDS unit at San Francisco General Hospital, an organizer of the conference, and one of the nation's most prominent AIDS doctors. "You just want to sit down and do the science, but that isn't always possible."

Perhaps there has never been a medical conference where that statement could be considered more appropriate. Science seemed the least important topic on the agendas of many of the more than 10,000 delegates. Intense hostility to the U.S. government's policy restricting immigration among those infected with the human immunodeficiency virus hung like a dark cloud over the proceedings.

Volberding, usually reticent about involving himself in political disputes, attacked the government policy at a news conference. He and other leading scientists stated that without some change in federal policy, the United States has almost certainly hosted its last international AIDS conference. "It's not appropriate to hold the conference in countries that have restrictions," he said. "It isn't fair. It's discriminatory. And it won't do anything to stop the spread of HIV infection."

In a publicly released letter to Lars Kallings, president of the International AIDS Society, Harvard AIDS researcher Max Essex, who is supposed to chair the 8th International AIDS Conference in Boston in 1992, said that without some change in U.S. immigration policy, Harvard will withdraw as sponsor.

"The Harvard AIDS Institute has reluctantly concluded that Harvard cannot remain a sponsor of a conference dedicated to the elimination of AIDS as long as those who are infected with the AIDS virus cannot travel freely to our country," Essex wrote. "Unless it becomes clear that the restrictions will be lifted shortly, we will withdraw as sponsor for the next U.S.-based meeting and cancel the conference scheduled for Boston in 1992."

Some delegates wondered -- as they have at past meetings -- whether these conventions play an important role in scientific research. In the age of computers and fax machines, it is now possible, and commonplace, for researchers at Duke University in North Carolina, a biotechnology company in Boston and a laboratory in Paris to be in constant daily contact, making large gatherings of scientists far less necessary.

Increasingly, the international AIDS meeting is regarded as an amiable get-together where scientists can swap gossip, insights and theories in a sort of extended bull session.

But few participants would describe this week's events as "amiable." At least one government health official, assistant secretary for health James O. Mason, received death threats, according to federal officials. Security was tight, and government AIDS researchers including Anthony S. Fauci and Daniel Hoth of the National Institutes of Health were encouraged to stay at the Marriott Hotel, the conference headquarters. During his closing address, HHS Secretary Sullivan was forced to dodge condoms and wads of paper that were thrown at him by hecklers, many of them members of ACT UP. In the past few years, ACT UP has repeatedly attacked the federal government for what it regards as a slow response to the epidemic.

Despite the massive amount of information that is presented at international AIDS meetings, many researchers say that the political maneuvering and emotional rhetoric have eclipsed the science.

The almost continual demonstrations from the many activist organizations boycotting the meeting, including the Gay Men's Health Crisis, pushed many here beyond their usual level of grudging tolerance. One senior U.S. government AIDS official, who asked not to be named because he said he was under too much pressure from activists, said that his interest in the science was dissipated by the constant attack on the motives and methods he and other researchers have used.

"Maybe we are not perfect," he said. "But when you listen to ACT UP describe us as genocidal maniacs, you suddenly start to wonder why you are devoting your life to this problem, why you never see your family, and why you haven't just thrown in the towel."

ACT UP emerged as the key organization at this year's meeting, largely by skillfully drawing attention to its daily protests and by setting the antagonistic tone for the week. Contrary to early expectations -- some activists had predicted riots or a takeover of the Moscone Center, where the meeting was held -- there was no violence. But as recently as last year, many researchers felt that AIDS activists and the medical community were moving toward a common ground. For the moment, at least, those days seem over.

For one thing, ACT UP issued broadsides against what it called the "Gang of Five," prominent AIDS researchers who test and evaluate drugs and whom ACT UP accuses of having improper financial relationships with pharmaceutical manufacturers.

But what ACT UP is really attacking is the slow pace of development and approval of new drugs, a system that relies on contributions from academic institutions, government agencies and pharmaceutical companies in equal measure. Many researchers -- and even some activists -- say it is not fair to single out scientists such as the "Gang of Five" as a way of demonstrating their frustration.

"Activists are mistaken when they assume or at least publicly state that scientists do not care about them," said Anthony S. Fauci, director of AIDS research at the National Institutes of Health, in his closing remarks. "This is devastating to a physician or scientist who has devoted years to AIDS research, particularly when they see so many of their patients suffering and dying . . . It is particularly devastating and unfair when scientists of good faith and enormous talent are singled out and publicly named as scoundrels."