The past two days have brought two major developments in the effort to fight AIDS.

In Washington, the Food and Drug Administration Tuesday approved widespread clinical use of AZT, a drug that checked some symptoms of the disease in limited testing.

And in Atlanta, medical researchers announced yesterday that they may have developed another drug that works as effectively as AZT without many of its adverse side effects.

Neither drug promises to cure AIDS, but both appear to inhibit reproduction of the virus that causes the deadly disease, researchers said.

After small-scale tests showed that AZT, or azidothymide, appeared to increase the life expectancies of AIDS victims, including patients treated at three Harvard-affiliated hospitals, the FDA agreed to classify AZT as an "investigational new drug," making it available to thousands of patients nationwide.

The Burroughs Wellcome Company, of Research Triangle Park, N.C., which produces AZT, asked for FDA authorization for large scale clinical testing of the drug after controlled trials showed that it is beneficial to AIDS patients who have recovered from one or more bouts with Pneumocystis carinii pneumonia (PCP).

PCP is one of a host of opportunistic infections that attack many AIDS patients because their immune systems have already been weakened by the virus. Experts estimate that almost 50 percent of the nearly 12,000 living AIDS patients in the U.S. have had this rare form of pneumonia.

AZT's manufacturer will make the drug available free of charge to most AIDS sufferers who have had the pneumonia within the last 120 days, the company said.

A spokesman for the Harvard-affiliated Brigham and Women's Hospital said that the hospital plans to participate in the expanded tests, but it will be at least another week before doctors know how many additional patients may be treated.

Patients at the Harvard-affiliated Beth Israel and Massachusetts General Hospitals also participated in the original AZT trials and will continue to administer AZT, spokesmen said.

If AZT proves equally effective in the new larger tests, the FDA may approve it as a prescription drug by the end of the year, said Brad W. Stone, an FDA spokesman. The FDA has made AIDS drugs a top priority, expediting their approval, Stone said.

In addition to its benefits, AZT can cause substantial side effects, such as severe headaches and anemia, which results from the supression of blood cell production in the bone marrow. The long-term effects of AZT are unknown.

The side effect problem may be avoided if CS-85, the drug described yesterday by researchers at Emory University and the University of Georgia, is as effective in clinical tests on human beings as it appears to be in laboratory tests.

"CS-85 appears to be as effective as AZT, but at least 10 times--maybe up to 100 times--less toxic," said Dr. Raymond F. Schinazi, a virologist at Emory.

"The CS-85 results are encouraging, but as yet they haven't even been given permission to use it in people," Stone said.