A slim, 17-year-old prostitute stood motioning to drivers as they circled the Washington bus depot on New York Avenue one evening recently. The scene could have been a downtown area in almost any big city. Except that the prostitute was male and his customers, whether they knew it or not, were violating the federal Mann Act.

In a little-noticed 1978 revision to the Mann Act, Congress prohibited interstate transportation of males as well as females who are under the age of 18 for purposes of engaging in sexual activity for pay. Transportation for such purposes within the District also is prohibited.

Since last year, the Washington field office of the Federal Bureau of Investigation has used the law four times in homosexual cases. Three of the cases have resulted in convictions, and the fourth is to be tried Monday in U.S. District Court here.

An FBI spokesman said that he did not know if any other offices might have used the law in homosexual cases. FBI field offices in New York, Baltimore and Miami said they have yet to use the law in such situations.

Several underage male prostitutes said in interviews that their numbers have been diminished by the possibility that their customers may be arrested on federal charges. But Frank E. Kameny, a leader of the gay rights movement in Washington, contends that prosecutors are singling out homosexual offenders.

"You can go out any night and see any number of underage female prostitutes, and there are no prosecutions by the FBI under the Mann Act," Kameny said. "This is clearly prejudiced against gays."

Kameny acknowledged that offenses against underage females and heterosexual prostitution are prosecuted under local laws, but he said that the perpetrators and their customers seldom are sent to jail. Because of the penalties provided in the Mann Act, those convicted so far have received stiff sentences. The penalty can be up to 10 years in jail or a fine of up to $10,000, or both.

On the other hand, several gay rights leaders say they believe a law that seeks to prevent exploitation of underage males or females is beneficial.

"We are against any form of prostitution, be it homosexual or heterosexual," said Jerry Weller, deputy director of the Gay Rights National Lobby, which monitors legislation affecting homosexuals. "We support the civil and sexual rights of consenting adults."

"It's exploitation, period," said Jeffrey W. Levi, president of the Gay Activists Alliance, a Washington area homosexual political group, referring to cases involving underage male prostitutes. "You're not going to find me defending that."

Both Levi and Weller said that they do not know whether the law is enforced evenly against heterosexual and homosexual offenders. Nationally, the FBI obtained 12 convictions in the first nine months of this fiscal year under the provisions of the Mann Act, now known as the Protection of Children Against Sexual Exploitation Act. The bureau said that it does not have a breakdown of the number of male and female victims.

Theodore A. Shmanda, an assistant U.S. attorney who has prosecuted the cases here, said: "The FBI was directed to enforce the law, since the law was new. That's why the FBI started doing these cases. Congress wanted to know what enforcement was being done. I don't see any differentiation between homosexual and heterosexual offenses , except that we have given more attention to the amendment because of its newness and the Justice Department's directive that the new statute be enforced."

In pursuing cases, the FBI makes use of tape recorders concealed on agents and physical surveillance, methods that the local police often do not have the time to use. Even with such aids, Shmanda said, prosecuting the cases is difficult because the children change their stories.

"It's heart-rending to see these kids who are 14 and 15 and have seen more sordid life than I've seen as a prosecutor in 10 years," Shmanda said. "You want to try to somehow help the kids by prosecuting the adults who are manipulating them. Yet for most of these kids, you're not really going to help them because they may continue on the wrong side for the rest of their lives."

In the most striking case so far, Timothy M. McNamara, 23,of Arlington and Philip Herbert Garrett, 25, of Adelphi were convicted of transporting a 12-year-old boy from Baltimore to Washington to engage in sex with a man who turned out to be an undercover FBI agent.

Posing as a Florida businessman, FBI Special Agent M. Glenn Tuttle met Garrett outside the Naples Cafe on New York Avenue NW and told him he was interested in a "young boy," according to court records.

Garrett later arranged for Tuttle to get in touch with McNamara, who, he said, "handles that age group," the court records show. Under FBI surveillance, McNamara went to Baltimore, picked up the 12-year-old boy, and brought him to Tuttle's hotel room in Washington. McNamara demanded $300, according to Tuttle, including $50 for the taxi ride from Baltimore. After receiving the cash, McNamara gave the youth $20. Tuttle and another agent, who had been hiding in the closet, then arrested him.

The two men were sentenced by U.S. District Judge John Garrett Penn to five years in prison. They cannot be released until they have served at least 20 months.

In other cases that have not been prosecuted because of insufficient evidence against the suspected perpetrators, the underage boys have said that they were given LSD or other drugs before being asked to perform sexual acts, according to law enforcement sources.

The reason for the emphasis on victims who are minors is that they are not sufficiently experienced to make informed decisions.

"Usually, there is a reward or pressure for the sex, and that is what locks the child in," said Ann W. Burgess, a professor of nursing at the University of Pennsylvania who has treated victims of such abuse.

"You see them start dropping out of school and withdrawing from their friends," added Burgess, who testified on behalf of the new federal legislation. "They're programmed for sex, and often they're given drugs, and they have to maintain whatever they're on by engaging in prostitution ."

Tim, the 17-year-old prostitute outside the bus station, said that he ran away to New York from his Gettysburg, Pa., home when he was 13 and was picked up by an older man who became his "sugar daddy." For the past three years, he has been visiting Washington, and six weeks ago moved here permanently.