Stick your big toe in first.

Don't demand to reopen the baths. Demand a study to measure the effect of baths that already exist.

Do bathhouses and sex clubs prevent AIDS? That might seem nuts, but it's the claim of a group of San Francisco activists who are lobbying to bring back the baths. They say commercial sex saves lives. I say it's time to call their bluff.

A group called Community United for Gay Sexual Privacy (CUSP) is trying to place an initiative on the November ballot that would reopen San Francisco's fabled baths as well as allow private rooms in the city's sex clubs. Both are currently forbidden by a ban dating back to 1984, when AIDS transmission was rampant.

But, say the CUSPers, that was then, and this is now. True, most researchers and public health officials argue that bathhouses and unregulated sex clubs help spread HIV, but the folks in CUSP say they know better. They're so confident that a thriving, unregulated commercial sex scene would actually prevent new infections that they call their proposal -- no kidding -- the STD Reduction Initiative Nondiscrimination Policy.

If this seems odd, the bottom line is that it's easy for CUSP to make such claims because no one is conducting studies that would definitively prove things one way or another. Sure, there are plenty of studies showing that the baths did play a huge role in HIV transmission back in the '80s but not what their effect is today. That's because AIDS cohort studies mostly look back in time, telling you what happened years ago, not what's happening now.

But a remarkable new HIV test called the "detuned ELISA" can change all that. The detuned ELISA identifies recently infected people, and researchers say they can now conduct real-time studies that will show, once and for all, the effects of modem-day baths and sex clubs on the AIDS epidemic. Unfortunately, no one is conducting such a study, so if CUSP is really devoted to "STP reduction," here's a suggestion: Don't demand to reopen the baths. Demand a study to measure the effect of baths that already exist in places like New York and Los Angeles.

Here's how such a study would work, according to Ron Stall of the Center for AIDS Prevention Studies at the University of California, San Francisco, one of the wisest voices in prevention research. In the past, when somebody tested positive you couldn't tell when they had become infected, and therefore you usually couldn't say how. Could have been three months ago, with a steady boyfriend. Or maybe two years ago, tweaking on the circuit.

But the detuned ELISA identifies those who have seroconverted just within the past six months, so the window is much, much narrower. Combining this tool with classical epidemiological case control design, researchers say they can now determine exactly where the epidemic is going -- whether rates are going up or down, precisely who is getting infected, how they're getting infected, and where the hot spots are. So we finally have the the means to answer a host of troubling questions about HIV transmission, including the precise risk of unprotected oral sex (long a matter of uncertainty and passionate argument) and the risks posed by unregulated sex clubs and bathhouses.

By establishing ongoing studies in cities that already have baths, researchers could quickly see if there's a preponderance of new infections among bathhouse and sex-club attendees and whether or not such venues are becoming hotspots of a resurgent epidemic.

And, hey, you never know. Maybe there is no preponderance. And if riot, I say let the good times roll. In fact, if attendees are found to be even less likely to seroconvert than nonattendees, why not reopen baths everywhere in the name of STD reduction"?

But -- and here's the big but -- if baths arid sex clubs are found to be hot spots, fostering new infections and a resurgent AIDS epidemic, then groups that claim to care about AIDS prevention would be honor-bound to stop new ones from opening and to campaign to shut down the ones that already exist.

If CUSP members are so genuinely devoted to prevention that they call their ballot initiative an "STD reduction measure, I'm sure they'll readily agree to such a plan. Gee. I wonder why they didn't think of it.