A worker in an AIDS research laboratory associated with the National Institutes of Health has become infected with the AIDS virus, the first reported case involving a research worker, federal health officials disclosed yesterday.

The worker was infected at least a year ago while handling a concentrated sample of a viral strain that scientists have identified as identical with the one found in the worker's blood.

"There is no question in our minds this virus came from the laboratory that person worked in," said Dr. Peter J. Fischinger, deputy director of the National Cancer Institute. "We used highly specific tests to isolate the strain and compare it to the lab virus. It is incredibly unlikely the infection could have come from any other source."

Health officials said this was the first documented spread of the AIDS virus they could be certain came from a specific laboratory. They did not reveal the person's sex, job status or place of residence, they said, because they were unable to reach the worker yesterday.

Fischinger would say only that the person worked at the National Institutes of Health or a laboratory working on a contract with NIH.

He said he did not know whether the infection was a result of a single accident or whether the person followed the strict precautions laid out by the federal Centers for Disease Control for handling highly infectious viruses in the lab. He said the lab where the infection occurred employed rigid procedures and specialized equipment for people working with dangerous organisms.

Fear of contracting AIDS has risen sharply among health-care workers and laboratory researchers since May 21, when the CDC reported that three women working in hospitals had been infected with the AIDS virus, known as HIV, after each was accidentally splashed with the blood of infected patients.

Officials sought yesterday to assure all those handling the AIDS virus that the disease cannot be contracted through casual contact. The virus can be passed only through blood or bodily fluids. The overwhelming majority of AIDS cases are the result of sexual contact or intravenous drug abuse. The virus also has been passed in blood transfusions and from infected mother to fetus. The only other documented cases have been caused by infected blood splashing abrasions or the accidental swallowing of blood or bodily fluids.

"Nothing has changed in how AIDS is passed from person to person," said Jim Brown, a spokesman for the Public Health Service. "We do not know how this individual became infected, but we believe lab workers are safe if they follow the procedures that are now in place."

Epidemiologists at NIH have tested hundreds of lab workers for AIDS the past three years and this is the only known positive test result, Fischinger and other health officials said yesterday.

The first indication that the worker was infected came a year ago, but the tests were inconclusive. After more recent tests showed that the person was infected, scientists interviewed the subject and began to try and isolate the virus in order to compare it with that in the lab.

Isolating the virus is a tricky procedure involving minute molecular examination. The process is called restriction enzyme mapping. It relies on enzymes to cut the genes in the virus into strips. Because different strains of the same virus do not have the same same sequence of genetic information, the enzymes will cut the virus in different places.

Scientists then compare the lengths of the resulting fragments. The only viral strain that was identical to those of the patient was the strain the worker had used in the laboratory.

Researchers were able to match the two virus samples only last week, officials said yesterday.

Federal health officials said yesterday that other lab workers at NIH have not yet been told of the incident but will be briefed as soon as the initial investigation is completed. Secretary of Health and Human Services Otis R. Bowen released a statement reporting the incident yesterday in response to reporters' inquiries.

Officials said that the worker had not developed symptoms of the disease, which disables the body's immune system. Many people infected with the virus do not show symptoms for years. Researchers now think that most of those infected will eventually show some symptoms.

Fischinger said that at least half a dozen labs at NIH are working with concentrated amounts of the AIDS virus thousands of times stronger than that found in humans.

He said he was concerned about safety guidelines in the labs because a growing number of researchers now work with the virus. The lab where the worker was employed was classified as P3, where the second most stringent level of precautions are used in federal labs.

The only level higher, P4, requires specialized research buildings and use of pressure controlled labs so that air is vented into the atmosphere through controlled circumstances.

"We intend to conduct a rigorous examination of our procedures," Fischinger said. "More people than ever before are working with HIV. And they need to follow the right precautions."

He stressed that the concentrated form of the virus commonly used in research labs would never be found in hospitals or associated with patients. High concentrations are necessary for researchers who must use a large quantity of the virus in their work.