In April 1972 -- nearly 18 years before a contaminated batch of the "food supplement" L-tryptophan killed 27 Americans and poisoned as many as 5,000 in one of this past year's worst public health disasters -- the Food and Drug Administration issued a warning.

L-tryptophan and other "health food" supplements made of isolated amino acids, the agency said, were potentially dangerous. The FDA banned sale of the pills unless they underwent a lengthy federal safety review.

The 1972 regulation should have ended the sale of L-tryptophan tablets then and there, preventing the fatal outbreak years before it occurred. But that never happened. Distracted by other matters and unsure of its chances for success, the FDA made a few efforts to enforce its ban and then dropped the matter.

Health food manufacturers, many of them fully aware of the regulation, simply ignored it.

By the time the L-tryptophan tragedy began to unfold late last year, few if any of the hundreds of thousands of Americans taking the chemical -- most seeking its alleged power as a sleeping potion -- knew that their pills had been declared unsafe and unfit for sale by federal health officials nearly two decades ago.

In more than a year of public investigation of the epidemic of death and disease linked to the pills, neither the industry nor the FDA has made any mention of the checkered history of the supplement. But according to public health experts, the fact that L-tryptophan remained on the market even after it was banned represents the true cause of the disaster.

"L-tryptophan was an accident waiting to happen," said Richard Wurtman, a professor of neuroscience at the Massachusetts Institute of Technology.

"If after 17 years no one was prosecuted for violating this law, then no one in industry is going to pay any attention to it," said Sidney Wolfe, director of Public Citizen Health Research Group. "That's why this disaster occurred."

L-tryptophan gained notoriety late last year when thousands of Americans who had bought the chemical came down with the crippling disorder eosinophilia-myalgia syndrome (EMS), which can cause fever, swelling, severe muscle pain, paralysis and -- in some cases -- death. Early evidence of the link between L-tryptophan and the rare disease was gathered by the Albuquerque Journal, which later was awarded a Pulitzer Prize for its work. Last March, all products based on the supplement were recalled by the FDA.

In the past few weeks, the outbreak has been traced to a specific contaminant found in a batch of the pills made by the Japanese firm Showa Denko. According to government investigators, the contamination appears to have been the result of a new and inadequately tested manufacturing process.

This has led some members of the dietary supplement industry to call the L-tryptophan incident an isolated problem. But a review of the supplement's history, according to some public health experts, reveals that it should never have been on the market in the first place.

L-tryptophan is a natural chemical, one of the 20 amino acids that, in various combinations, make up all forms of protein. Each of these amino acids has markedly different chemical structure and function, and for many years it has been known that if taken individually, these chemicals can have valuable properties.

The artificial sweetener aspartame, for example, is a form of the amino acid aspartic acid. The flavor enhancer monosodium glutamate is a form of the amino acid glutamine. L-tryptophan, for its part, if taken in large enough doses that it manages to reach the central nervous system, has been shown useful in treating insomnia and -- in large doses -- depression.

Since these were all chemicals continuously consumed by humans as components of dietary protein, the FDA originally had no qualms about their manufacture and sale. In fact, throughout the 1960s, L-tryptophan and all the amino acids were designated by the FDA as "Generally Recognized as Safe" and placed on the "GRAS list," which is a comprehensive list of foods like sugar or flour that are considered so risk-free they need no prior federal approval before they can be processed and sold.

But in the early 1970s the FDA began to have second thoughts. New scientific evidence suggested that amino acids consumed individually had markedly different physiological effects than when they were eaten in food. In particular, the agency began to worry about the possible health effects of consuming larger doses of individual amino acids than the body was accustomed to.

"Experimental animal studies have shown that the adverse effects of {taking larger than average amounts of amino acids} are sub-optimal food intake, growth retardation and degeneration of certain organs which can lead to the animal's early death," the FDA stated in its 1972 Federal Register notice on amino acids. "This indicates a potential health risk for man if use is not limited."

For L-tryptophan itself, the list of potential health risks is extensive. According to medical experts, it should not be take by pregnant women, diabetics and people with bowel problems. In laboratory rats, it has been linked with bladder cancer. When taken with some antidepressant medications, it also can cause shivering, mildly manic behavior and can also worsen the drug's side effects.

None of this information was included on labeling of the L-tryptophan tablets sold in health food stores.

On the basis of this evidence the FDA in the early 1970s proposed removing all amino acids from the GRAS list. To sell amino acid supplements, the new regulation stated, a manufacturer would have to file a formal petition proving that they were safe and showing how they would be made and used.

This regulation, if enforced, would have meant that the contaminated Showa Denka L-tryptophan would never have been sold in the United States. Even if L-tryptophan manufacturers had followed the regulation and sought formal approval, drug law experts say, the manufacturing processes used to make the supplement would have been strictly regulated, making a contamination unlikely.

For example, Canada, which treats L-tryptophan as a prescription drug and tightly controls its production and sale, had only 12 cases of EMS, and all were apparently caused by pills purchased in the United States.

But the FDA's attempt to enforce its 1972 rules against the unregulated sale of L-tryptophan was completely unsuccessful. The initial problem was that because of an agency error, L-tryptophan was inadvertently left on the published GRAS list even after the regulation had been issued taking it off. Because of that technicality, the FDA's first major action against an L-tryptophan maker in the mid-1970s was reversed in the courts.

After correcting the error, the agency again moved against the industry in 1977. But that action also ran into trouble in the courts and, faced with a seemingly interminable legal battle and a judge that FDA officials describe as hostile, the agency abandoned that suit in 1982.

Thereafter, the agency apparently made only sporadic attempts to inform supplement makers of its disapproval, sending letters reminding them of the rules regarding amino acids. But no further concrete action was taken.

At the same time, agency officials say they were dissuaded from pursuing the case against the amino acid makers because of the special legal protections that Congress gave the supplement industry.

In an unusual 1976 law sponsored by then-Sen. William Proxmire (D-Wis.), Congress gave the makers of vitamin and mineral supplements a status enjoyed by no other industry. The Proxmire Amendment prohibits the FDA from taking any regulatory action against the manufacturers except in cases where a clear public health risk exists. It is not entirely clear that amino acids fall under the protection of the law. But the agency apparently felt that they would, believing that the precedent established by the act would doom aggressive actions against amino acid supplements to failure.

"The problem is that the Proxmire umbrella is hanging over all of this," said Alan Forbes, who until last summer was director of the FDA's Office of Nutrition and Food Sciences. "The amendment essentially says that the FDA should do nothing with respect to essential nutrients unless you have dead bodies in the streets. That puts the agency in a bit of a box."

Industry officials, for their part, say they were well aware of FDA's intention to remove amino acid supplements from the market. But they appeared to pay that no heed.

"I wouldn't quibble with anyone who said that the intellectual mindset of the FDA is that they would like to assert that these are not legal substances," said J. B. Cordaro, head of the Council for Responsible Nutrition, the main food supplement industry trade group. "But their ability to make that stick has not been borne out in the real world."

Even though there is a regulation outlawing amino acid sales, Cordaro said the initial mistake leaving it on the GRAS list, the two failed legal actions by the FDA, and the agency's reluctance to enforce its own rules left the industry with "no other position to take" than that L-tryptophan was still legal.