They have not been able to find it again.

The discoverer, William Fenical of the Scripps Institution of Oceanography, has returned repeatedly to the Philippine island of Siquijor in a vain search for more of the chemical.

''We've been looking for years, and still have never found it again,'' he says. ''We find some creatures that look a lot like it, but none of the animals there had the right compound.''

Researchers don't even know if the creature itself produced the compound, or if it was a byproduct of some symbiotic fungus or bacteria.

But the story, reminiscent of the 1992 fictional movie ''Medicine Man,'' in which Sean Connery finds - and then loses - a cure for cancer in the Amazon, may have a happy ending.

Researchers at the University of California-Santa Cruz saved some of the sample, and within a few years they expect to synthesize the compound, called diazonomide A.

Fenical's team, working on a National Cancer Institute grant, made their discovery in 1991.

They collected samples of a rare creature called Diazona chinensis, a jellylike animal related to the sea squirt, which attaches itself to rocks and filters its food from the ocean.

The creature had been studied two or three decades earlier, but showed no promise.

This time, laboratory tests turned up something new.

''Much to our surprise and shock, there were enormous amounts of this new molecule, and the molecule had the ability to kill human colon cancer cells with very high proficiency,'' Fenical says. ''It was very potent at very small doses.''

He emphasizes that such test tube success doesn't mean the compound works in humans. That can only be determined by clinical trials, and many times such compounds prove too toxic for medical use. But the lab tests were exciting.

Diazonomide A was also exotic structurally, with an unusually rigid, compact form that interested chemists. Fenical sent some to chemistry professor Joseph Konopelski at UC Santa Cruz.