ST. JOHNSWORT IS an attractive but annoying plant that works as a reverse sunblock, so when sheep and cattle eat it they tend to get nasty sunburns. Besides that, the plant thrives anywhere, and if it weren't for heroic pest-control measures it would long ago have taken over Utah rangeland.

But it is the weed's irritating heartiness that may prove a boon to humans.St. Johnswort is just one of hundreds of plants that scientists in Utah and elsewhere are now examining in the hopes of coming up with new drugs to treat everything from AIDS to the common cold.

The reasoning is this: If a plant is resistant to disease and bugs, maybe it contains chemicals that would protect humans too. Maybe, in fact, the cure for our most elusive and deadly diseases has been right under our noses all along.

From the deserts of Utah to the rain forests of Samoa, botanists are chopping off leaves and bark and sending them to laboratories to be analyzed, tested and, occasionally, turned into prescription drugs.

And plants aren't the only ingredients in nature's medicine chest. University of Utah scientists are studying extracts from sponges and other marine organisms scooped up from coral reefs in the South Pacific (see story on this page).

After decades in which American pharmaceutical companies preferred to design drugs from scratch, there is now renewed interest in finding help from nature, according to Brigham Young University chemist Steve Wood.

"Plants are wonderful chemists," he says. "They come up with chemicals you wouldn't ever think to make in a laboratory."

Scientists, for example, have been trying for years to invent a drug that is effective against ovarian cancer. Now there is hope that the Pacific yew - an unassuming tree that grows in the shade of Douglas firs in the Northwest United States - may contain a compound, taxol, that succeeds where chemists couldn't. The National Cancer Institute of the National Institutes of Health is currently testing taxol in clinical trials.

According to Gordon Cragg, chief of the institute's Natural Products Branch, the NCI hopes eventually to test 10,000 to 15,000 species of plants against 60 different types of human cancers. It has also tested about 12,000 plant extracts, from 2,000 plants, against the HIV virus. The results are "very preliminary," Cragg says, but have produced "some interesting leads."

Of the thousands of plant extracts studied, only 5 percent show any form of activity against disease, Cragg says. "To get a winner like taxol" the odds are more like 1 in 100,000, he says.

Developing a new medicine, from the time a plant is first studied to its eventual transformation into a wonder drug, takes years. Scientists have been trying to unlock the secrets of the Pacific yew plant since the early 1970s.

At BYU, chemist Wood works with virologists Byron Murray and James North, ecologist Rex Cates and biochemist Bronwyn Hughes. The group studies plants ranging from local aspen to exotic Panamanian flora, testing their anti-viral properties plus their possible use as natural pesticides and insecticides.

St. Johnswort, for example, may prove promising against cytomegalovirus, a herpes virus that can cause fatal infections in people with weakened immune systems.

Of the world's estimated 250,000 different plant species, only 5,000 have been thoroughly investigated as sources of medicine. From those, 90 have been turned into some 120 drugs - including digitalis, morphine and L-Dopa.

Some of the plants scientists are now exploring have been plucked at random from forests all over the world. Others come from native healers, by way of ethnobiologists like BYU's Paul Cox (see related story on C1).

"We're on the crest of a wave now," says Cox. "Three to five years from now there's a chance we'll come up with a new Vincristine."

Vincristine, a drug that has proved effective against leukemia, was developed in the 1960s when the Eli Lilly company began studying 40 plants from Madagascar. The Madagascar periwinkle plant, they discovered, contained compounds that could kill certain cancer cells.

But when Lilly and other drug companies began testing other plants they weren't so lucky. Vincristine, they decided, must have just been a fluke.

"So at the end of the '60s, the doors of the drug companies came crashing closed" to botanists, says Cox.

In the past five years, however, plants have become more fashionable again, partly in response to fears that the world's rain forests are disappearing, partly out of desperation to find a cure for AIDS.

But the United States still lags far behind other countries in the study of medicinal plants, says Wood. "We're kind of a third-world country in this regard."