Laboratory experiments announced Friday show that a plant toxin combined with a synthetic protein is able to attack and kill cells infected with the AIDS virus while leaving healthy cells untouched.

In a study published Friday in the journal Science, a group of researchers report that a manmade molecule called recombinant CD4 can be used to deliver a killer toxin to cells infected with AIDS in test-tube experiments.However, they cautioned that the new laboratory development - like other incremental advances in AIDS research - was far from being ready for clinical application and, at best, might offer a potential new treatment approach rather than a cure.

Dr. Jonathan W. Uhr, chairman of the department of microbiology at the University of Texas Southwestern Medical Center in Dallas, said the CD4 molecule naturally binds to a gycoprotein, called gp120, that forms on the surface of cells infected with human immunodeficiency virus, or HIV, which causes AIDS. CD4, he said, will not attach itself to cells not infected with HIV.

In the laboratory, Uhr said, his research group combined a synthetic CD4 molecule with a toxin called ricin that is extracted from plants.

When exposed in a test tube to HIV-infected cells, he said, the CD4-ricin combination binds to the cell surfaces and the ricin then kills the cell, thus eliminating a source of HIV virus.

"Early in the course of HIV infection, T-cells and macrophages (two types of immunity cells) are thought to be the major cellular reservoirs for the virus. Both of those bear the CD4 molecule that permits the virus to enter," said Uhr.

By combining a toxin with the CD4 molecule, the researchers are, in effect, creating a guided missile that attacks only those cells that are infected. Healthy cells are not touched by the toxin, said Uhr.

The scientist said toxic effects of the CD4-ricin combination will have to be studied in the laboratory before the technique could be used in clinical experiments. Ricin has been used in cancer drugs and has been found to be well tolerated by patients if managed carefully, he said.

Uhr said it will be at least a year before the drug could be tested on patients. And, though this technique shows promise, he said he is not suggesting that it would cure AIDS.