Interferon, a virus-fighting substance, backfires in some cases to make a virus infection much worse instead of better, according to a new study by the National Institutes of Health.

Researchers found that in one strain of mice attacked by a virus called lymphocytic choriomeningitis, interferon treatment made the viral attack so much worse that the mice quickly died.

Dr. Robert M. Friedman, who did the study at the NIH with Dr. Charles Pfau of Rensselaer Polytechnic Institute, said that the interferon proved to be "too much of a good thing." Interferon is a natural body substance that has raised much hope in the treatment of viral diseases and cancer. It has been extremely scarce, but is now available in enough quantity that numerous tests of the substance are going on in America and Europe.

The new test result should not hamper this research, since this negative effect appears to be very rare, Friedman said.

He said the virus infected the brain of the mice, and then would have spread throughout the body in a mild version of the disease.

But instead, the interferon worked effectively to prevent the spread of the virus to the rest of the body. So, the infection became bottled up in the brain. The body's immune reaction to the infection concentrated all its force in the brain, inflaming the tissue around the brain and central nervous system to fatal proportions.

Friedman said that there are human diseases caused by viruses of the same family, the arena viruses, but that they affect people in Central and South America almost exclusively. He said he did not know whether interferon would have the same effect in those diseases, or whether any other strain of virus might react the same way.