Paying the price for sex with minors. TEENS IN TROUBLE: IN SAN FRANCISCO ...

Fewer cars are cruising lower Polk Street in San Francisco these days, a strip where teen-age runaways and street kids are known to strike deals of sex for sale with the men behind the wheels. The reason for the slowdown in teen prostitution here? One explanation lies in the headline-grabbing indictment in October of 14 people involved with a teen-sex ring operating in San Francisco and nearby Foster City.

``There's definitely less activity on the streets than before [the indictments],'' says Jed Emerson of Larkin Street Youth Center, a private agency that works with troubled teens to try to get them off the streets. ``With all the media attention, no one wants to see their car and their license plate flashed across the evening news in connection with a child-prostitution story.''

Cracking a prostitution ring is not usually big news in most US cities, but the San Francisco case is unusual. First, it exclusively involved teen-age girls, most of them runaways, as young as 14. Second, the prosecution focused not only on the prostitutes and the pimps, but also brought felony criminal charges against eight brothel patrons, including a former top city official and a San Francisco police officer.

Prosecuting the patrons, or ``johns,'' represents ``a real step forward'' in establishing some parity in the criminal-justice system, Mr. Emerson says. While it is illegal for anyone to engage in sex-for-sale transactions, often only the prostitutes are arrested while the johns are not, he says.

``The reason this prosecution is good is that we finally are seeing who these guys are,'' he says. ``And they are not dirty old men in trench coats, like we all like to think. They are leaders in our community, the next-door neighbor, politicians, police, your husband.''

Persuading law-enforcement agencies to prosecute adult patrons as well as child prostitutes is a herculean task currently being undertaken nationwide by the National Center for Missing and Exploited Children in Washington.

``Ten years ago if the police came upon a deal, they would have locked up the hookers and hustlers - the kids,'' says John Rabun, deputy director at the national center. But the center's training efforts with police, district attorneys, juvenile-court judges, the FBI, and social workers are slowly changing the official response to teen prostitution, he says.

Still, only seven states have laws that specifically target the patrons of juvenile prostitutes, ``a fact not highly surprising in view of the fact that over 20 states impose no punishment at all on patrons,'' according to the center's 1987 report on child pornography and prostitution. Further, ``in only five states is the patron of a juvenile prostitute subject to greater punishment than the child exploited,'' the report notes. ``In fully half the states, he is actually subject to less punishment.''

``It is logical that a child should not be held as accountable as an adult, but too often the kid pays and the adult does not,'' Mr. Rabun says.

``It is important that society up the ante in terms of adult males who think they can go out willy-nilly and abuse kids. It's so incredibly easy right now for adults to get away with misusing kids,'' he says.

Apparently, no reliable data exist on the number of juvenile prostitutes in the United States. But the center's 1987 report, written in conjunction with the US Justice Department's National Obscenity Enforcement Unit, says an estimate of between 100,000 to 300,000 children involved annually in prostitution is ``highly justifiable.''

Experts do know, however, that the estimated 1 million teen-agers who run away from home each year are the youngsters most at risk of getting involved in prostitution and child pornography. Most kids on the run procure food and shelter either by trafficking in stolen goods, by selling drugs, or by prostitution - and many teens perceive prostitution to be the easiest way to make money while avoiding the long arm of the law, Rabun says.

Law-enforcement officers and prosecutors say it is extremely difficult to crack prostitution cases because most prostitutes will not implicate their pimps and customers. It is even harder to crack juvenile prosecution rings, they say. Police say they are often hamstrung because they cannot get the evidence they need without involving a minor who can get hurt in the process.

In the San Francisco case, however, an 18-month investigation turned up young prostitutes willing to testify against the brothel operators and the customers, says deputy district attorney John Carbone. Although attorneys for some of the defendants have charged the prosecution was politically motivated, Mr. Carbone says the case proceeded because ``the inclination of the young girls to be cooperative made it easier to develop the identities of patrons and to seize evidence.''

Last month, Roger Boas, former chief administrative officer of the city and a candidate for mayor last year, pleaded guilty to having sex with teen-age prostitutes. Although his plea bargain kept him out of jail, he was sentenced Nov. 18 to six months' labor in a county alternative- work program and was fined $100,000.

Police officer Patrick Miyagishima, who also entered a guilty plea on similar charges, received a one-year suspended jail term, was placed on 18 months' probation, and was fined $1,000. Of the six brothel operators, three are in custody pending arraignment and three are at large.

The prostitutes in this case have not been charged because of their cooperation with the investigators.