A modified version of a natural poison kills AIDS-infected cells in the test tube while ignoring normal cells, offering a possible strategy to help treat AIDS, scientists reported Thursday.

The specialized toxin may be able to kill infected cells before the cells produce more viruses, said researcher Bernard Moss of the National Institutes of Health.Moss, Ira Pastan and five other NIH scientists describe experimental results in Thursday's issue of the British journal Nature.

In a second development, scientists at NIH and in Italy say they have produced stable AIDS virus infections in rabbits, a step that may help animal testing of AIDS vaccines and drugs. Until now, such infections have been shown only in chimpanzees, which are costly and in limited supply.

The poison research involved Pseudomonas exotoxin A, which is produced by bacteria called Pseudomonas aeruginosa. It is so strong that a dose about the size of a salt grain can kill a human.

"We're encouraged both by the low amount of drug which is effective" and its ability to ignore uninfected cells in test-tube experiments, said Moss.

To make the poison seek out AIDS-infected cells, researchers essentially replaced the part of the poison molecule that normally guides the toxin to its target. In its place they put part of a protein called CD4, which latches onto a protein found on the surface of AIDS-infected cells.

The redesigned poison was produced by combining genes for the poison and the CD4, and placing them into bacteria called E. coli.