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LOS ANGELES — from www.nytimes.com – Before they take off all their clothes, the actors who perform as James Deen and Stoya go through a ritual unique to the heterosexual adult film industry.

First, they show each other their cellphones: Each has an e-mail from a laboratory saying he or she just tested negative for H.I.V., syphilis, chlamydia and gonorrhea.

Then they sit beside the film’s producer, Shylar Cobi, as he checks an industry database with their real names to confirm that those negative tests are less than 15 days old.

Then, out on the pool terrace of the day’s set — a music producer’s hilltop home with a view of the Hollywood sign — they yank down their pants and stand around joking as Mr. Cobi quickly inspects their mouths, hands and genitals for sores.

“I’m not a doctor,” Mr. Cobi, who wears a pleasantly sheepish grin, says.

“I’m only qualified to do this because I’ve been shooting porn since 1990 and I know what looks bad.”

Bizarre as the ritual is, it seems to work.

The industry’s medical consultants say that about 350,000 sex scenes have been shot without condoms since 2004, and H.I.V. has not been transmitted on a set once.

Outside the world of pornography, the industry’s testing regimen is not well known, and no serious academic study of it has ever been done. But when it was described to several AIDS experts, they all reacted by saying that there were far fewer infections than they would have expected, given how much high-risk sex takes place.

“I don’t think there’s any question that it works,” said Dr. Allan Ronald, a Canadian AIDS specialist who did landmark studies of the virus in prostitutes in a Nairobi slum.

“I’m a little uncomfortable, because it’s giving the wrong message — that you can have multiple sex partners without condoms — but I can’t say it doesn’t work.”

Despite the regimen’s apparent success, California health officials and an advocacy group, the AIDS Healthcare Foundation, are trying to make it illegal to shoot without condoms. They argue that other sexually transmitted diseases are rampant in the industry, though the industry trade group disputes that.

In January, the city of Los Angeles passed a law requiring actors to wear condoms. A measure to do the same for the whole county is on the ballot on Tuesday.

Producers say the condom requirement will drive them out of business since consumers will not buy such films. Local newspapers like The Los Angeles Times oppose the ballot measure, calling it well-intentioned but unenforceable, and warning that it could drive up to 10,000 jobs out of state.

Very frequent testing makes it almost impossible for an actor to stay infected without being caught, said Dr. Jacques Pepin, the author of “The Origins of AIDS” and an expert on transmission rates. “And if you are having sex mostly with people who themselves are tested all the time, this must further reduce the risk.”

When the virus first enters a high-risk group like heroin users, urban prostitutes or habitués of gay bathhouses, it usually infects 30 to 60 percent of the cohort within a few years, studies have shown. The same would be expected in pornography, performers can have more than a dozen partners a month, but the industry says self-policing has prevented it..

“Our talent base has sex exponentially more than other people, but we’re all on the same page about keeping it out,” said Steven Hirsch, the founder of Vivid Entertainment, one of the biggest studios.

Performers have to test negative every 28 days, although some studios recently switched to every 14.

If a test is positive, all the studios across the country that adhere to standards set by the Free Speech Coalition, an industry trade group, are obliged to stop filming until all the on-screen partners of that performer, all their partners, and all their partners’ partners, are found and retested. In 2004, the industry shut down for three months to do that.

It has had briefer shutdowns in each of the last four years.

In 2009 and 2010, no other infected performers were found. Coalition representatives said an infected woman in 2009, from Nevada, may have had an infected boyfriend, and offered evidence that a man infected in 2010 in Florida had worked outside the industry as a prostitute. The 2011 test was a false positive.

A shutdown in August came after several actors got syphilis, not H.I.V. All performers were given a choice: Take antibiotics, or pass two back-to-back syphilis tests 14 days apart.

State law in California covering these issues is vague. It requires protective gear for workers exposed to blood-borne diseases, but it was written with nurses and police officers in mind and does not mention condoms.

The California Occupational Safety and Health Administration interprets the law as requiring condoms, but says it cannot find the industry’s workplaces — most movies are shot in houses rented for a few days. Even if it does, it cannot enter without the producer’s permission or unless a performer files a complaint, which would allow investigators to get a warrant, said Amy Martin, the agency’s chief counsel.

But the industry hires only actors willing to work without condoms, so complaints are not filed.

The industry says the state’s interpretation would require all films to be shot with latex gloves, face shields and lab coats. It also says the state tolerates other industries, like boxing and football, in which performers take risks.

The Legislature has avoided making the law specific to pornography.

“No politician wants to touch the issue, because it’s about sex,” said Michael Weinstein, head of the AIDS Healthcare Foundation, who has made the issue a personal crusade.

For producers, it’s really about money.

Vivid Entertainment shot with condoms for two years after a 1998 H.I.V. outbreak, and sales dropped 30 percent, Mr. Hirsch said. Producers have threatened to leave the state, taking the jobs of 1,200 actors and more than 5,000 crew members with them.

Because of court rulings, filming adultmovies is legal in California, Florida and New Hampshire. Nevada, where prostitution is legal, tolerates it.

Gay porn, by contrast, has included condoms since the 1980s, because producers assume some actors are infected and because many gay men consider forced testing an invasion of privacy.

However, that is changing as “bareback” films become more common. Some producers claim their actors are safe because they shoot in places like Slovakia, where H.I.V. rates are very low, or because they test, or because infected actors are taking antiretroviral drugs, which can reduce the risk of transmission by 96 percent.

But it remains controversial. Chi Chi LaRue, who has been producing gay pornographic movies for 28 years and shoots only with condoms, dismissed bareback producers as “industry pariahs.”

While cases of H.I.V. are rare, Mr. Weinstein and California health officials say they also want condoms required to stop other diseases.

Thirty five diseases — including drug-resistant gonorrhea — can be transmitted sexually, and the industry tests for only four, said Dr. King Holmes, head of global health at the University of Washington.

Los Angeles County officials estimated that porn actors get chlamydia and gonorrhea at least eight times as often as other young adults. The industry disputes that estimate, but a new study by the county suggests that the industry’s urine testing for gonorrhea underestimates how much is transmitted through oral and anal sex.

Actors insist that they are careful because their jobs depend on it. Top performers can earn $200,000 to $400,000 a year.

“If I get gonorrhea, we have to cancel the shoot, the crew is angry at me, and that’s unprofessional,” said Stoya. “And besides, it’s gonorrhea — yecch. So I use condoms in my personal life.”

Her co-star, Mr. Deen, concurred: “If I’m having sex off camera for fun, and it’s not someone from the industry who tests all the time, then it’s condoms, condoms, condoms all the way.”

Several working and former actresses interviewed said they opposed condoms for another reason: They chafe.

Actors repeatedly stop and start while lights and camera angles are changed and different versions of scenes are shot.

“The average length of intercourse for most Americans is 10 minutes,” said Nina Hartley, a former nurse and well-known porn actress since 1984. In porn, she said, “it’s 30 to 60 minutes of thrusting. It doesn’t matter how much lube you use, it’s uncomfortable, it’s a friction burn, and it opens up lesions in the genital mucosa. I could handle two to three condom scenes a month. But actors are paid by the scene, and I couldn’t do three in a week.”