Researchers Thursday reported the first "very promising" evidence outside a test tube that an experimental new treatment will slow the lethal spread of the AIDS virus inside the body.

The medicine, CD4, apparently works as a decoy, mimicking the blood cells that the virus usually infects and kills.In a report in Thursday's journal Nature, researchers reported that the treatment dramatically reduced the level of virus in monkeys infected with the simian version of the AIDS virus.

"This is the first evidence in vivo (in a living animal) that it has anti-viral effects with chronic administration in a relevant animal model," said Dr. Robert Schooley of Massachusetts General Hospital.

"That still leaves the question of whether or not there will be parallel positive effects in humans," he said. "But it does increase the enthusiasm and the likelihood one would push on with more extensive studies."

Recently, Schooley and others began testing CD4 on people infected with HIV, the human AIDS virus, but the results will not be known for weeks.

"This approach looks very promising for AIDS," said Dr. Norman L. Letvin, senior author of the monkey study.

The treatment cannot wipe out the virus when hidden inside the body's cells. However, if it works as researchers hope, it will absorb loose virus that is roaming the bloodstream looking for new cells to take over.

The latest research, conducted at the Harvard-affiliated New England Regional Primate Research Center, suggests that CD4 will do just that. It showed that when given to infected rhesus monkeys, the drug dramatically reduced levels of the virus.

Even if the treatment works as well in people as it does in monkeys, it will not cure AIDS. But it might make life better for victims of the virus.