Chimpanzees in the wild appear to practice herbal medicine.

Moreover, according to scientists who have discovered this behavior, the leaves on which the chimps rely have been found to contain a powerful antibiotic that shows promise as a drug for humans.

The leaves that chimps methodically seek out and swallow, apparently when ill, contain a substance that laboratory studies show to be a potent killer of bacteria, fungi and nematodes -- all of which can cause serious diseases in apes and humans alike.

The discoveries, which have emerged gradually, began some 20 years ago when Jane Goodall, a pioneer chimpanzee researcher, found that chimp dung often contained one species of leaf that invariably had not been chewed.

Subsequently her colleagues at Tanzania's Gombe National Park discovered the source of the leaves -- a bushy shrub called Aspilia that grows 6 to 10 feet tall -- and observed the animals' behavior when seeking the leaves.

"Perhaps chimpanzees will show us a drug that can one day be used in the Western world," Richard Wrangham, an anthropologist at the University of Michigan, wrote in an account of the discoveries in AnthroQuest, the newsletter of the L.S.B. Leakey Foundation, named for a pioneering anthropologist.

At Gombe, chimps seek out the leaves as soon as they wake up in the morning. Instead of breakfasting at the nearest source of wild fruit, which is their usual practice, some chimps will walk for 20 minutes or more to open, grassy areas where Aspilia bushes grow.

Instead of promptly tearing off the leaves and eating them, the animal will gingerly close its lips over the unplucked leaf and hold it for a few seconds. Several leaves are tried in this way before the chimp selects one and places it in the mouth.

Instead of chewing, the ape rolls the leaf around in the mouth for perhaps 15 seconds and then swallows it whole. Over perhaps 10 minutes the chimp may select and swallow up to 30 small leaves.

The Gombe researchers found that while chimps of all ages and both sexes will use Aspilia, females do so more often, averaging about one day in 10. Males take the leaves about one-third as often. Another oddity is that while Gombe chimps swallow the leaves mainly in the morning, those at another national park do so at any time of day.

Chimp researchers have long speculated on the special role of the leaves, wondering if it were an intoxicant or hallucinogen. Part of the answer emerged last year.

Eloy Rodriguez, a biochemist at the University of California-Irvine, began searching for unusual chemicals present in Aspilia leaves and quickly isolated a previously unknown substance. It was a red oil that was subsequently named thiarubrine-A.

Coincidentally another scientist, Neil Towers at the University of British Columbia at Vancouver, had discovered the same substance in Canadian plants a few weeks earlier. In a search for medicinal properties, Towers found that thiarubrine-A was an extremely powerful antibiotic that could kill common disease-causing bacteria in concentrations of less than one part per million.

Rodriguez and Towers, who were friends, shared their findings and touched off a renewed investigation of Aspilia's possible use as a medicinal plant. They did one study that showed the chemical contents of the leaves probably being released in the chimp's digestive tract. Electron microscope views of the leaves, removed from chimp dung, showed that surface cells had been ruptured, apparently during passage through the gut.

Speculating that the plant's usefulness might already be known among African peoples, the two, along with Wrangham, visited herbaria and research centers in East Africa. They found evidence that, indeed, Africans knew of the plant's value and used its leaves to treat wounds and stomach aches.

Of the four species of Aspilia, chimps use the leaves of three. African peoples, it turned out, use only the same three. "It is beginning to look as if chimpanzees and people have similar ideas about Aspilia leaves," Wrangham wrote.