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Last summer's drought left a legacy of poison that threatens the country's food supply. Dry weather spurred the spread of the fungus that makes aflatoxin, a highly potent inducer of liver cancer. Dangerous amounts of aflatoxin contaminate a fifth or more of the Middle West's 1988 corn crop, much of which is still in storage. Food companies and governments have multilayered defenses in place to screen against the poison, but these now face a vigorous assault.

The crux of the issue is that the many consignments of corn found to be contaminated are not destroyed but simply rejected. The poisoned corn, as chronicled in a recent Wall Street Journal report, goes back into the pool available for market, and most seems to be consumed one way or another.

Some of the contaminated corn goes to feed beef cattle. The poison is mostly destroyed in the body, but the meat still needs to be checked for aflatoxin residues.

Some of the corn goes to dairy cattle, and dangerous amounts of aflatoxin may pass into the milk. Illinois, Iowa, Minnesota, Missouri and Texas have already had to dump contaminated milk.

Some corn is shipped abroad. Since aflatoxin is such a well-known problem, it's the responsibility of importers to have shipments tested according to national food laws.

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And some corn, maybe after dilution, is presumably offered again to makers of food products like breakfast cereals, corn bread, corn muffins and hominy.

The scale of the problem is vast. The Middle West provides a third of the world's corn, and a third of test samples in Iowa and Illinois have been found to contain dangerous amounts of aflatoxin, as judged by the Food and Drug Administration's standards. Most years the Quaker Oats cereal plant in Cedar Rapids, Iowa, rejects between 1 and 2 percent of corn shipments; this year's weekly rejection rate runs between 20 and 35 percent. The level of contamination in the 1988 crop thus appears to be at least 10 times greater than usual. What are the chances that the poisoned part of the crop is being redistributed without endangering the food supply?

The main assurance lies in tests conducted by food companies at every stage from raw grain to finished product. They have a strong economic incentive: recall of a product because of unacceptable aflatoxin levels could cause heavy losses.

The F.D.A. also tests raw corn, milled corn and finished food products. So far, none of 71 finished products tested since October have been found to exceed the danger level for aflatoxin, and the F.D.A. believes, though is not certain, that the products were made from 1988 corn. If so, the defenses in place seem to be working so far.

But it's impossible to test every item on the supermarket shelves, and so much poisoned corn is in circulation that it might penetrate chinks in the screening system like small manufacturers who may not thoroughly test their corn purchases.

Minute amounts of aflatoxin are an inevitable ingredient of food, and an extra burden of poison for one year need not necessarily be disastrous. To cause cancer, most carcinogens would have to contaminate a person's diet for years. But any increase in a chemical as potent as aflatoxin is worrisome.

The F.D.A. has already stepped up its aflatoxin tests and its regional inspectors have the authority to test grain in storage. Increased testing by companies and regulators is the least that is required. If that fails to restrict the poisoned corn to cattle feed, more vigorous measures had better be taken against farmers and grain elevator operators who freely traffic in poisoned lots. Corn products and milk seem safe at present, but only special vigilance will contain the threat.

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