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The same thing happened when they gave the mice influenza infections. Those animals that made MIP-1alpha developed nasty lung infections with severe congestion, making it difficult for them to breathe. But the mice that could not produce MIP-1alpha had lungs that were nearly clear.

With both viral infections, Dr. Smithies said, the immune response caused "more problems than the virus itself."

Dr. Oppenheim said that MIP-1alpha also is involved in autoimmune diseases, when the immune system attacks a person's own cells and organs. For example, he said, when the chemokine is blocked with antibodies, mice no longer can develop a disease that resembles multiple sclerosis.

Dr. Jerome Groopman, an AIDS and blood researcher at Deaconess Hospital and the Harvard Medical School in Boston, has been studying MIP-1alpha's effects on bone marrow cells. In the course of some viral infections, including hepatitis, the bone marrow can shut down, causing a life-threatening disorder, aplastic anemia. "It was suspected that MIP-1alpha could be a mediator," Dr. Groopman said. When he added the chemokine to bone marrow cells growing in the laboratory, it killed them, he discovered.

Dr. Krensky, who is the discoverer of Rantes, said that he and others have found the molecule at the sites of a wide variety of diseases, including rheumatoid arthritis, endometriosis, multiple sclerosis, lupus, some cancers and even atherosclerosis. It is also present where transplanted organs are rejected. In fact, Rantes seems to be present so often that "it almost becomes a joke when you go through these diseases," Dr. Krensky said.

Rantes stands for "Regulated Upon Activation Normally T-Expressed and Presumably Secreted." "I made a horrible mistake," Dr. Krensky said. "I let my students name the molecule."

Immunologists say they are not certain what would happen if someone tried to treat H.I.V. infections by injecting patients with chemokines.

Dr. Groopman said he suspects that it might be a disaster because chemokines are meant to work locally in the body. If they fight H.I.V., they do so in the immediate area of an infection. "It's probably a matter of a measured dose of poison," he said.

Dr. Krensky said the same. "We have Rantes -- we could do it tomorrow. Shoot it into a mouse and see what happens," he said. But, he added, "the phenomenon is local."

Dr. Oppenheim said that when he injected chemokines into mice, it actually diminished their immune responses. The reason is that the circulating chemokines destroyed the body's ability to notice chemokines that were released locally. So, Dr. Oppenheim said, treating AIDS patients with chemokines "could make them more susceptible to infections."

On the other hand, immunologists said, it might be possible to stimulate white blood cells to step up their own local production of chemokines to fight H.I.V., rather than to simply swamp the body with injections of chemokines.

And, they said, the idea that chemokines might actually have antiviral effects is so new that no one can say what would happen if the chemicals were tests as an AIDS treatment.

"I don't think enough is known to make predictions," Dr. Oppenheim said. "I am sure these agents will have many many side effects. But they need to be evaluated."

Photo: Chemokines may cause problems worse than infections they fight. (The New York Times) (pg. C3) Diagram: "Signposts for Recruits in the Immunity Wars" shows how chemokines marshall immune cells from the blood and guide them to the site of infection. (Source: Dr. Alan M. Krensky/Stanford University)