The Environmental Protection Agency yesterday slapped new restrictions on the nation's most popular corn and soybean herbicide and began a special review to determine if the chemical should be banned as a potential carcinogen.

The chemical, called alachlor, is made by the Monsanto Co. and is most commonly sold under the brand name Lasso.

More than 90 million pounds of Lasso and other alachlor-based herbicides are used each year to control weeds in U.S. corn and soybean fields, making it the nation's No. 1 agricultural herbicide and one of the most profitable products in Monsanto's line of farm chemicals.

Citing recent studies that showed alachlor caused at least four types of tumors when fed to animals, the EPA said it was taking immediate action to reduce the risk to farmers who use it. The agency estimated that about 600,000 farmers and farm workers face cancer risks as high as one in 10,000 from alachlor exposure.

Under the new rules, alachlor users must wear protective clothing, and aerial spraying of the chemical will be forbidden. Use on potatoes also will be banned.

In addition, Monsanto will be required to label the product with a warning that it causes cancer in laboratory animals, and will have to foot the bill for an educational program designed to train farmers to use the chemical safely.

David Crosson, a spokesman for the St. Louis-based company, said Monsanto did not expect "any major disruptions" as a result of the new controls. He also said that the company did not expect the EPA's investigation to lead to a ban of the chemical.

"It is a major product for Monsanto, and it's also a major product for corn and soybean growers," Crosson said. "Monsanto is confident that after the special review, registration will continue."

But EPA officials said yesterday that there are still unanswered questions about the chemical, including the potential risks of alachlor residues in the food supply.

The chemical is widely used on grains intended for human and livestock consumption, and "it is certainly likely that it is present at some level in meat, milk and poultry," said Paul Lapsley, head of the agency's special pesticide review section.

Although the chemical has been marketed since 1969 and is now used on about 30 percent of the corn and soybeans grown in the United States, Lapsley said the EPA has almost no information on food residues.

Alachlor also has been found in streams and underground water supplies in at least three major corn-producing states at concentrations of up to 267 parts per billion, and "exposure of humans . . . through drinking water is likely," the EPA said.

As a condition to the continued sale of alachlor, the EPA ordered Monsanto to conduct more than a dozen additional tests on the chemical, including studies on its ability to cause birth defects and genetic damage and a probe of residues in food.