Like doctors trying to isolate a virus before it causes an epidemic, U.S. Information Agency officials are seeking to halt a rash of unsubstantiated reports in the world press that Latin American slum children are being sold to provide organ transplants for wealthy U.S. buyers.

The story apparently originated in Honduras early last year and has popped up periodically since then in different parts of the world. Early this month, it was picked up by two international news agencies -- first Reuter and then Agence France-Presse -- and their reports have caused it to be circulated with increasing frequency in major newspapers throughout Western Europe and the Third World.

USIA officials say the situation is alarming not only because of the sensational, overtly anti-American aspects of the story but also because they believe its durability is partly due to the Soviet Union's "disinformation" propaganda apparatus, which seeks to spread false or misleading ideas about the United States.

As a test of its improving Soviet relations, the Reagan administration has pressed Moscow to cease its disinformation activities, and U.S. officials acknowledge that they have little evidence of recent Soviet efforts to circulate forged documents and other material aimed at fanning anti-American attitudes.

The slum-child story did not originate with the Soviets. But U.S. officials assert that when Soviet disinformation experts learned about it, they did not hesitate to spread it around.

"It's a retreat from their formerly crude methods to a subtle new approach where they hide behind the fact that the material didn't originate with them," said Herbert Romerstein, chief adviser on disinformation activities to USIA Director Charles Z. Wick. "They've done it with stories that the United States originated the AIDS virus for germ warfare purposes, and now they're doing it with the story that babies are being bought and dismembered for their organs."

Romerstein, once an investigator for the now-defunct House Internal Security Committee, has been denounced by liberals as a "red baiter" and praised by many conservatives as a tireless tracker of the subversive webs spun by communist agents here and abroad.

Two years ago, in what appeared to be an attempt to make him the butt of an "in joke" among intelligence officials, the Soviets allegedly circulated a phony letter purportedly written by Romerstein. The gist of the letter was that the United States should exploit the Chernobyl nuclear power plant disaster for propaganda purposes.

Romerstein and his assistant, Todd Leventhal, have compiled an extensive chronology demonstrating the history of the child-buying story and Soviet efforts to spread it.

The chronology has been sent to U.S. embassies and consulates throughout the world so they can use it to rebut versions of the story that have appeared in the local news media. And the USIA yesterday held a rumor-control briefing for foreign journalists stationed in Washington.

According to the record pieced together by U.S. officials, the story originated in January 1987 when a Honduras newspaper, La Tribuna, quoted a senior government official, Leonardo Villeda Bermudez, as saying there was organized trafficking in organs of small children sold by their parents. He immediately repudiated the story, saying he had made no charges but had casually mentioned "unconfirmed rumors."

However, the story spread in ensuing weeks to Guatemalan newspapers and then was picked up by the media in other parts of Central America and the Caribbean. The first evidence of a Soviet connection came on April 5, 1987, when Pravda published a dispatch from its Mexico City correspondent repeating the original Honduran reports of Villeda Bermudez's alleged charges but omitting his subsequent disclaimers.

"It wasn't a life of paradise and the home hearth of well-to-do Americans that awaited the Honduran boys and girls," the Pravda article said in part. "Swindlers supplied them as donors for sick and disabled children from rich families. They are broken down, or rather sold off, literally in parts . . . eyes, kidneys, hearts, everything that can be used for transplantation."

On the same day, the Pravda story was sent around the world by the Soviet news agency Tass. In subsequent months, it cropped up periodically in publications as far away as India, sometimes in communist-controlled newspapers such as L'Humanite in France or Barricada in Nicaragua, and sometimes in mainstream papers such as India's Hindustan Times.

For a time, repeated prompt and authoritative official denials prevented the story from gaining much currency. In July 1987, the European Community Commission stated that it had no information to substantiate allegations. U.N. Secretary General Javier Perez de Cuellar and Oxfam, the private international child welfare organization, issued similar statements saying the reports apparently were "fictitious rumors."

But the story was given a new lease on life by an Aug. 7 Reuter story from Asuncion, Paraguay, quoting a provincial judge there as claiming that kidnaped babies "were going to be killed at American organ banks." The judge's remarks later proved to be unsubstantiated variations on the story that originated in Honduras.

Two days later on Aug. 9, after upset U.S. officials had contacted Reuter and explained the situation more fully, the agency ran another story revealing the long history of the child-selling rumors and noting that the judge's charges were unsubstantiated.

In the meantime, however, the Aug. 7 report had rocketed around the world on the Reuter wire and had been carried by prominent newspapers in such countries as Britain, France, West Germany, Switzerland, Austria, Belgium, Portugal, Italy, Norway, Canada, New Zealand, Argentina and Brazil. Many of the stories were published with accompanying pictures of children allegedly bound for the United States to be killed for their organs.

At this point, U.S. officials charge, the Soviets entered the situation again hiding behind one of their surrogates -- the International Association of Democratic Lawyers (IADL), a Brussels-based organization that Romerstein and other U.S. intelligence officials contend is controlled by Moscow.

The IADL earlier had submitted reports to the U.N. human rights subcommission in Geneva, stating it had no evidence of the child-selling allegations but arguing without success that they should be investigated further. When the Reuter report hit Europe, the IADL resubmitted its reports to the subcommission and also released them to Agence France-Presse, which recycled the story once again.

"During the past two weeks," Romerstein said, "a tragicomedy of errors, ignorance and disinformation has taken a totally unsubstantiated, long-discredited rumor and catapaulted it throughout the world press as if it were a fact."