WASHINGTON (AP) _ The Food and Drug Administration said Thursday it will allow a potentially promising new AIDS drug to be tested on people infected with the deadly virus.

Researchers say the drug, known as GLQ223, is unique because it appears to kill only those immune system cells that are infected with the AIDS virus, leaving non-infected cells alone.

However, they caution that the drug so far has been tested only in the laboratory and has not been used on AIDS patients.

''I'm very optimistic because of the test tube results, but there's always the possibility it could have unpredictable side effects or when given to humans the drug may not be able to get to all the infected cells,'' said Dr. Michael McGrath of the University of California in San Francisco and San Francisco General Hospital.

McGrath led a team of scientists from UCSF, the Chinese University of Hong Kong and Genelabs of Redwood City, Calif., that developed GLQ223.

The drug is a highly purified form of the plant protein trichosanthin, which is derived from the root of a Chinese cucumber plant. The plant extract has been used in China to induce abortions.

In the laboratory, the drug has been found to work on two different types of cells crucial to the normal functioning of the immune system - T-cells and macrophage cells, which act as a reservoir for the virus.

AZT, the only drug now licensed for treatment of AIDS, and other drugs like it only prevent the virus from replicating in T-cells.

David Corkery, a spokesman for the American Foundation for AIDS Research, called the drug ''very promising,'' and said he is ''cautiously optimistic'' about its prospects.

Genelabs said Thursday the clinical trials are expected to begin in May at San Francisco General Hospital, primarily to determine safety and human tolerance of the drug. McGrath said the trials are expected to last three to six months.

If a safe dosage can be determined, then trials would be conducted on the effectiveness of the drug.

Genelabs also cautioned that though GLQ223's active ingredient is found in some Chinese herbal medicines, ''such preparations may also contain other components with unknown toxic effects.''

Also, the company said that since the toxicity of trichosanthin itself has not been evaluated in AIDS patients, it ''cautions against the use of trichosanthin or trichosanthin-containing preparations as an AIDS therapy.''

The drug is being developed with money from the Swiss chemical and pharmaceutical company Sandoz Ltd., which would have rights to market it.