A phantom chemist in California is foiling authorities by tinkering with molecules to create legal "designer drugs" that addicts say are as strong as heroin but that have killed at least 85 people.

"He's clearly a state-of-the-art chemist," said Gary Henderson, a pharmacologist at the University of California at Davis, who coined the term designer drugs. "He'd have to be or he'd kill himself making this stuff. It's that potent."

Drug laws define illegal drugs by their exact molecular structure, so it is possible for the chemist to evade the law by making minor changes in the substance's molecules.

By the time scientists decipher the structure of the chemist's latest product and enforcement agencies take steps to outlaw it -- which can consume several months -- the chemist simply redesigns the molecule.

Although all of the deaths have occurred in the West, drug officials and chemists say it is only a matter of time before designer drugs spread nationwide.

The mystery chemist is known to have produced 10 variations of the drug fentanyl, widely used as an anesthetic in surgery but otherwise illegal.

Fentanyl has the same effect as heroin or morphine, and addicts can switch freely back and forth, but it is far stronger than heroin and therefore must be cut to a greater degree for street sales.

Only two of the variations of fentanyl have been outlawed. Pharmaceutical industry researchers have produced an additional 210 variations, none of which ever reached the market, and hundreds more theoretically are possible.

"Whoever this guy is, he's obviously got some chemical training and he's got a good laboratory," said Donald Cooper, a chemist who has analyzed the phantom chemist's products at the U.S. Drug Enforcement Administration's laboratory near Tysons Corner, Va. "There are good indications he's cognizant of our legal efforts and he's trying to stay one step ahead of us."

Cooper and other chemists who have been tracking the phantom chemist since 1981, when his products first appeared, have developed a grudging admiration for him. They cite a drug he made called 3-methyl fentanyl. It is about 1,000 times stronger than heroin. The amount absorbed through the skin from a smudge on one finger would be "enough to knock you down," said Henderson.

But, Henderson said, "the stuff he makes is clean and the dose is cut just right. Every time I get a sample from the police, I think about this guy. I wonder what he's got in store for us. It's usually pretty interesting."

Henderson and Cooper are among many who believe it is inevitable that designer drugs will spread east. All the deaths so far have been in California except for two in Oregon and one in Arizona.

Although recipes for making fentanyls are not yet in the cookbook form used by clandestine drug labs making LSD, PCP, amphetamines and other older drugs, Henderson and Cooper worry that a chemically sophisticated person could figure it out from chemical literature.

All but one of the fentanyls made by the phantom chemist are forms described in the chemical literature. One, called para-fluoro fentanyl, was his own invention. Investigators say they believe that only one chemist is likely to be at work because the same combination of other substances is used to cut the drug. This would be unlikely if several chemists were working independently.

"If this thing breaks out of the West Coast, we hate to think what could happen," said Ronald Buzzeo, deputy director of the DEA's office of diversion control. "We don't have the legal apparatus to really go after this kind of thing. We're looking at ways to control these drugs as a class but the lawyers and the scientists say it's not clear we can."

The problem is how to write a definition broad enough to include the whole fentanyl family without being so broad that courts consider the law too vague. "Besides," Henderson said, "no matter how many variations you put in the law, I'll bet I can think up another one."

Although several classes of designer drugs are made in other clandestine labs, fentanyls are the major concern now. Another group, for example, also acts like heroin but causes Parkinson's disease.

Because plain fentanyl is about 80 times more powerful than heroin, dealers usually cut it far more than they do heroin. It is often sold as China White, the usual name for a superior grade of Asian heroin. The 3-methyl fentanyl form is sometimes sold as Persian White, the finest grade of heroin.

Plain fentanyl, like all organic compounds, is a molecule made of several kinds of atoms -- carbon, hydrogen, oxygen and nitrogen -- linked in a specific configuration of rings, strings and other patterns. The shape of the resulting molecule and the nature of the atoms at various positions determine how it will react in the body.

Designer alterations of fentanyl are possible, however, because some points on the molecule can bind to additional atoms or groups of atoms. In the case of 3-methyl fentanyl, the molecule is a plain fentanyl with a methyl group (a carbon surrounded by three hydrogens) stuck on at a position that chemists, by convention, designate as No. 3.

For reasons not understood, this alteration makes the molecule react much more strongly with the nerve cell's opiate receptor sites, multiplying its strength. The alteration also turns the molecule into something other than fentanyl, something legal.