Federal safety rules and practices protecting laboratory workers from the AIDS virus need to be reconsidered after study of how one worker became infected, according to a report in Science magazine.

Despite earlier reports by officials at the National Institutes of Health that the worker had violated safety guidelines, the new study said that the infected worker was careful and followed guidelines, but became infected anyway.

Officials said the Centers for Disease Control have begun drafting revisions of the safety guidelines to take the case into account. The revisions are expected to be released soon.

The case, at first a mystery to researchers, is the only one known in which a laboratory worker became infected in the ordinary course of research on acquired immune deficiency syndrome, without being exposed to infected blood or stuck with a needle containing the virus.

The worker -- who has not been identified -- was one of 265 lab workers who volunteered for a study to determine the safety of working with the virus. At first, despite denials of the worker, some officials believed that the infected worker must have acquired the disease through sexual relations and not from laboratory work.

However, the worker apparently had no history of high-risk sexual behavior and had not received any suspect blood transfusions.

The worker also seemed to have followed standard safety precautions for working with live virus, including wearing a lab coat and gloves and working at a bench under a protective hood.

The AIDS virus is highly changeable, having many different strains, however. In the Jan. 1 issue of Science, researchers report that they have demonstrated the infection must have taken place at work -- the strain that infected the worker was identical to the strain the worker handled in the lab.

"We found no specific violations of the safety guidelines," said Dr. Stanley H. Weiss of the New Jersey Medical School, chief author of the report in Science. Still, the worker became infected, and "this raises questions about the guidelines. We may have to be much more strict," he said.

For example, workers now wearing one pair of gloves perhaps should wear two, because occasionally workers noticed pinholes or small tears in gloves after they worked with virus.

In addition, lab workers should be tested regularly for signs of the AIDS virus and should be made to review correct procedures.

Weiss said it is not known for certain how the worker became infected. But there were occasional leaks of virus-containing liquid from a laboratory centrifuge, and though masks and gloves were worn during cleanups, it is possible that some spills went unnoticed.

As a result, the worker might have handled equipment not known to be contaminated. Then, the worker's gloved hands may have inadvertently brushed against mouth, nose or other exposed skin. In the lab experiments, the fluids being worked with contain extremely high concentrations of virus, millions of times that found in humans.

In the Science paper, the researchers said that although one infected worker was found, the infection rate among lab researchers is very low. Even among those exposed to concentrated virus, the rate is less than one infection for every hundred person-years of work.

Still rare but somewhat more common -- about a dozen cases have been reported -- are infections of clinical workers who have accidentally injected themselves with virus-loaded blood, or who have splashed themselves with significant quantities of infected blood.

The Science paper also reported the case of a lab worker who apparently became infected after an apparently contaminated needle cut the worker's hand.

The researchers emphasized that in both cases the workers were being careful, but each needed to be more strict in carrying out the general guidelines.

The article recommended regular testing of workers' knowledge of the guidelines with special attention to such practices as removing gloves after contact with any possibly contaminated machinery even when contamination seems unlikely, avoiding all contact of hands with eyes, nose, ears or mouth, and careful decontamination of equipment.

Also recommended were regular checks of lab machinery to ensure no inadvertent contamination is taking place; any leakage should trigger a formal review of procedures and a check of workers.