A University of Minnesota researcher has identified a chemical toxin, or poison, made by the bacteria that cause toxic-shock syndrome -- a discovery that health officials say should make it possible to diagnose the dangerous disease by a laboratory test.

The toxin, discovered by Dr. Patrick Schlievert, an assistant professor at the University of Minnesota Medical School, represents "the first really good lead" in the quest to identify the strain of bacteria responsible for the illness, according to Dr. Bruce Dan of the federal Centers for Disease Control.

Disease experts at the centers alerted the nation last year to an outbreak of the rare but sometimes fatal illness, which most often strikes women during their menstrual periods. Evidence that toxic-shock syndrome was caused by a new strain of staphylococcus bacteria prompted the search for some way to differentiate the strain from others that do not cause the disease.

Dan said the new toxin "is an important identifying marker for this bacteria." He said it is produced by almost all staphyloccus bacteria found in thevaginas of toxic-shock victims, but only rarely by other strains of "staph" from healthy women.

Schlievert's toxin is a chemical released by staph bacteria that have been isolated from women with the disease. Injected into laboratory animals, it produces many of the symptoms characteristic of toxic-shock syndrome in humans, such as fever, diarrhea, a dramatic drop in blood pressure and a sunburn-like rash.

Dan said that using the toxin, researchers will work to develop a test that can detect the presence of chemical antibodiees against the toxin in a patient's blood. Such a test could prove that the patient had toxic-shock syndrome, which until now has been defined by a collection of symptoms that also occur in other diseases. Schlievert believes the test would identify many mild cases of the illness that are being missed.

Schlievert would not reveal the details of his research, which will be published in a medical journal this spring. Although nearly all reported cases have occurred in women, Schlievert predicted that once a laboratory test for toxic-shock syndrome is developed, as many as 20 percent of the cases of the disease will be diagnosed in men and children.

"Don't be misled into thinking that women are the only ones who get this disease," he said.

The federal centers have received reports of 992 cases of toxic-shock syndrome in the United States, 77 of them fatal. Last month the centers reported that the number of cases seemed to have peaked last summer and had been declining steadily since then. The decline was linked to a decrease in the proportion of American women using tampons -- from 70 percent last July to 55 percent last December -- following the discovery that tampon use increased a woman's risk of the illness.

The fall in reported cases also coincided with the withdrawal from the market of Procter & Gamble's Rely tampons, which had been particularly implicated in the disease. Dan said that in several studies of toxic-shock syndrome, between half and three-quarters of the victims were user of Rely.

However, some disease experts disagree that the incidence of the disease is declining, and blame the decrease in cases reported to the centers on relaxed surveillance. "We've seen absolutely no decline in toxic-shock syndrome in Minnesota," said Dr. Michael Osterholm, state epidemiologist. "Our [figures] are identical for all four quarters of last year," he said, and cases have been reported at the same rate so far this year.

"In those states where they're looking for them [cases], they're still occurring," he said.

Osterholm reported in January that a study of toxic-shock victims in all three states had implicated not only Rely, but all high-absorbancy tampons in promoting the illness. He said that further analysis of tampon absorbency has confirmed that finding and that he expects to release a list of tampon brands ranked according to their absorbency in a few weeks. According to his study, the tampons with the lowers absorbency -- and hence apparently the lowest risk -- are Tampax Junior and Tampax Slender Regular.

A spokeswoman for Procter & Gamble said the company believes that the withdrawal of Rely is not responsible for the decrease in cases of toxic-shock reported to the centers, and pointed out that the centers' figures for the final months of 1980, after the withdrawal, are similar to the totals for the first several months of 1980, when Rely was still being sold.

"All we're saying is, toxic-shock syndrome has really not gone away," she said.

Changes in the tampon market have prompted other manufacturers to reformulate their products. A spokesman for Kimberly-Clark, manufacturer of Kotex tampons, said the company has removed a super-absorbent chemical from its products "because consumers no longer showed a preference for tampons containing superabsorbent material." He added that in the last few months the company's tampon sales have dropped moderately while its sales of feminine napkins have increased substantially.

A spokesman for Tampax Inc. said, "We're going to reintroduce a tampon we first sold in 1936," one that was withdrawn less than two years ago to make way for a more absorbent produce. Made of pure cotton, the remarketed tampon will be called "Original Regular."

"The Centers for Disease Control and the Food and Drug Administration have not indicated . . . that anything is the matter with the material in our tampons or anyone else's," the spokesman said. "But . . . it behooves us to provide the customers with what they want."

According to health officials, women can greatly reduce the chance of getting the rare disease by not using tampons. For those who prefer to use tampons, the risk can be reduced by not using them continuously during menstrual periods. Any woman who develops a high fever, abdominal pain, vomiting or diarrhea while using a tampon should remove it and consult a doctor. Women who have had toxic-shock syndrome should receive antibiotics to prevent a recurrence and should discontinue tampon use.