The "tracking agent" that the State Department said yesterday was being used to monitor the activities of U.S. diplomats in Moscow appears to be unknown among forensic chemists and other specialists in the use of science for surveillance.

The Soviet powder, chemically called nitro phenyl pentadiene aldehyde, is not listed in any standard chemical reference book.

Although special powders are used by police in this country to mark valuables, preliminary indications were that the Soviet powder was of a different sort -- one that may persist longer on the skin or in clothing and one that may elude detection by the usual means.

U.S. "marking powders," which glow under ultraviolet light, the so-called "black light," are used by police to mark money and other valuables so that thieves may pick up smudges that are visible only under ultraviolet light.

Several chemists said the molecular structure of the Soviet tracking agent suggested it might be similarly fluorescent, but a State Department official maintained that it was not and that the powder could be detected only by subjecting it to laboratory analysis.

Some forensic scientists suggested that although fluorescent powders would be easier to use in tracking people, they would also be easy for the marked person to detect and wash off.

The State Department also said the substance caused mutations in bacteria in the Ames test, a standard method of screening chemicals for cancer-causing potential.

But all indications are that Moscow embassy personnel picked up quantities too small to be hazardous. Dr. Charles Brodine, a State Department specialist in environmental health, told embassy employes that it could be measured in quantities of only a few billionths of a gram.

"Substances that are positive on the test," said Bruce Ames, the inventor of the test, "are often carcinogenic but there's no way you can tell without further tests." Ames, a professor at the University of California in Berkeley, said the amounts reportedly used probably pose less of a health threat than a variety of other substances naturally present in foods and which do cause cancer in large doses.

A cup of coffee, Ames said, contains natural carcinogens in amounts far larger than those of the substance picked up by embassy personnel.

The State Department said specialists from the National Institutes of Health and the Environmental Protection Agency would go to Moscow to evaluate the degree of exposure and that their findings would not be available for some time.

When Brodine briefed several hundred embassy employes and family members last night he said "any danger is far from proved" and added that tests so far "all argue that the level of risk is fairly low." Many said it was the first they had heard of the powder and several expressed concern about possible health effects, especially about the danger to pregnant women.

"I have been here two years now and I want to know what it is I should do," one young mother told Washington Post correspondent Celestine Bohlen.

Another embassy employe after the briefing said people were "concerned but quite understanding that the information at this point had to be incomplete."

Several forensic experts in this country said they knew of no comparable use of marking powders to track people. "Marking powders are used by security firms to mark valuables but to my knowledge there's no chemical substance that's put on a person to tell who he's in contact with," said John Hicks of the FBI laboratory in Washington. "But it does sound perfectly conceivable that this could be done."

Another forensic scientist, Walter F. Rowe of George Washington University, expressed skepticism about the State Department's announcement.

He recalled a previous incident in which the State Department claimed the Soviet Union was spraying a toxic "yellow rain" in Southeast Asia. The substance turned out to be bee feces, he said.

"After the yellow rain business," Rowe said, "I don't have a bit of faith in the State Department. I don't say it's impossible the Soviets are doing it. I just say I would like to see a lot more evidence."

Attempts by The Washington Post to learn more about the Soviet powder from various federal agencies with expertise in such areas were unavailing.