SAN FRANCISCO (AP) _ In the age of AIDS, the Chicken Ranch brothel is an island of safe sex in a sea of disease.

Researchers spent most of the last year examining the results of 4,858 gonorrhea cultures, 1,333 blood tests for syphilis and 849 HIV antibody tests.

The preliminary results, scheduled to be presented at the Sixth International Conference on AIDS on Saturday: 20 positive gonorrhea tests, all but one before 1985; no positive syphilis tests since 1983; and no positive HIV tests since AIDS antibody screening began in 1986.

That was the same year condom use became the law at all 35 legal brothels in the state of Nevada.

The study was conducted by researchers from the School of Public Health at the University of California, Los Angeles.

''I think this represents a successful marriage - wrong term - a successful collaboration between private industry and state regulators in the state of Nevada,'' said Dr. Gary Richwald, assistant professor in UCLA's Department of Community Health Services, who led the $21,000 study funded by the Nevada Brothel Association.

The wrong results could have meant disaster for the ranch, but it was a risk co-owner Russell Reade was willing to take.

''Along about 1985, when the AIDS epidemic really hit and was really a news item, I really seriously thought that perhaps the party was over,'' said the former high school biology teacher who has co-owned the ranch since 1982. ''But with the protections we offer with the mandatory testing and the mandatory condoms, I was pleasantly surprised that business increased, and we've actually had an increase every year.''

Every week, the 21 ladies of the Chicken Ranch are screened for gonorrhea; HIV and syphilis tests are done monthly. All tests are state law. There have been no positive HIV results in any of Nevada's 300-400 licensed prostitutes.

''I require that the working ladies of the Chicken Ranch go directly from the lab where the test is administered to the Chicken Ranch and wait for the test results after returning from any time off whatsoever,'' Reade said. ''We agree that a lady is in complete control in the brothel to mandate the condom. She may not be able to exercise such complete control in her personal life, and there is where she is probably at greater risk.''

From 1982 to 1989, about 87,000 men visited the Chicken Ranch, in Pahrump, Nev., about an hour outside of Las Vegas. Only 31 of the 7,000 men who came to the brothel in the first half of this year refused to use condoms.

''They're gentlemen about it. They just happen to have very strong feelings about it,'' Reade said. ''They are graciously refused service.''

''For many of the men coming to the Chicken Ranch brothel, they're using condoms for the first time,'' Richwald said. ''I think this demonstrates condom use is possible, successful, can be intimate and is a behavior they might consider at home as well.''

The ranch's employees tend to be white and in their 20s. All fees are negotiated, and most women see about 15 clients a week for a gross weekly income of about $3,000. Half of that goes to the brothel.

The owners make sure their employees know the latest techniques to make safer sex erotic.