More than a decade after "safe sex" messages began to curb an AIDS epidemic that was crippling the gay community, scientists are seeing disturbingly high rates of new infections and risky sexual practices among young homosexual men. Early results gleaned from blood tests and interviews with 1,781 men in Florida, Texas and California show that 7 percent were infected with the human immunodeficiency virus, which causes AIDS. More than a third of those surveyed, all 15 to 22 years old, said they had engaged in unprotected sex within the previous six months. "These figures are alarming because these young men grew up in an era of HIV awareness," said Dr. Linda Valleroy, an epidemiologist with the U.S. Centers for Disease Control and Prevention. She was speaking at the annual convention of the American Association for the Advancement of Science, being held in Baltimore. The findings suggest the educational messages that altered sexual practices of the baby-boom generation have not sufficiently filtered down to the younger group. The people surveyed were not yet in their teens when the epidemic surfaced in the early 1980s and created a crisis atmosphere. Through June 1995, when the latest U.S. figures were published, nearly a half-million people had been diagnosed with AIDS. The government estimates that between 630,000 and 900,000 Americans are infected with HIV. Dr. Valleroy and other experts reviewing trends in the AIDS epidemic said new campaigns targeting teen-agers and young men will be needed if the nation is to avoid another surge of illness among the homosexual population. The survey, which began in 1994, also shows vast differences among ethnic groups. Infection rates were almost three times higher among young black gays than among whites -- 11 percent compared with 4 percent. The rate was 7 percent among young Hispanics. The survey has been conducted in Dallas, Miami, Los Angeles, San Francisco and two neighboring Bay-area counties. In a few weeks, researchers at the Johns Hopkins School of Hygiene and Public Health will make Baltimore the first Eastern city to be surveyed. Dr. Alfred Saah, director of a long-term study of the AIDS epidemic among gays in Baltimore, said he has recruited a team of young gay men to conduct interviews and draw blood samples. They will attempt to recruit volunteers outside bars, coffee houses, restaurants and businesses frequented by gays -- mostly in the Mount Vernon section of town. They will operate out of a mobile van. A curtain drawn across the center of the vehicle will provide separate spaces to interview two men at a time. Similar techniques and venues were used in the other cities. "The lore is that gay men have taken care of the infection within their own group," Dr. Saah said in a telephone interview. "Well, that's not true. There are currently new infections going on, just lower than it would be in a drug-using community. "There is prevention work to be done in a group that is largely tabbed as having taken care of the problem." Dr. Saah said he wasn't surprised by the results, but called them "astronomical." "They ought to be a fraction," he said. "Based on what we know today, 7 percent is too high because we've got all the prevention strategies. We know how it is transmitted, and we know how to prevent it." The virus is spread principally by semen during unprotected sex and by the exchange of tainted blood among drug addicts sharing hypodermic needles. "The infection rates among young gay men are a third to a quarter of what they were in 1984, when the virus was spread silently and nobody knew what they were dealing with," Dr. Saah said. That year researchers discovered the virus that causes AIDS; it was another year before a blood test was available. In the six cities surveyed so far, 38 percent of the men said they had engaged in at least one act of unprotected sex within the previous six months. Rates of HIV infection were 5 percent among men 15 to 19 and 9 percent among men 20 to 22. Because of the virus's long incubation period, teen-agers who get infected tend to have full-blown AIDS by their late 20s and die in their 30s. The CDC decided to conduct a survey of American cities after public health authorities in San Francisco reported disturbing trends among gay men 17 to 22. A survey done there in 1992 and 1993 found that 9 percent of the participants carried the AIDS virus while 33 percent said they had engaged in unprotected sex.