Aventis SA said it is suspending seed sales of the genetically modified corn that tainted Kraft Foods' taco shells.

The French pharmaceutical and agricultural company, inventor of the corn, said it won't resume U.S. sales of the seed until Washington approves the crop for use in human food, which regulators say won't happen anytime soon. It is the first time a major crop-biotechnology firm has frozen sales of a genetically modified seed.

Aventis, began licensing U.S. companies to market the genetically modified seed, called StarLink, in 1998 after the Environmental Protection Agency cleared the corn for use in livestock feed and for making ethanol fuel. The corn, engineered to make its own insecticide, isn't approved for human consumption. Regulators aren't convinced the bug-killing protein, called Cry9C, isn't a potential food allergen.

Kraft, a unit of Philip Morris Cos., New York, began voluntarily recalling millions of taco shells from U.S. supermarkets Friday after independent laboratory tests conducted for it confirmed a report from an antibiotechnology group that some shells illegally contain ingredients from StarLink corn. The shells are marketed under a license from Taco Bell, a fast-food chain owned by Tricon Global Restaurants Inc.

Aventis' move won't have noticeable financial impact on the company, which had revenue of $20.7 billion in 1999 on a pro-forma basis. Aventis, created by the December 1999 merger of European pharmaceutical companies Hoechst AG and Rhone-Poulenc SA, is generating roughly $1 million this year from licensing the fledgling StarLink gene to U.S. seed firms. In 4 p.m. New York Stock Exchange composite trading Tuesday, Aventis rose 44 cents to $74.44 a share.

Aventis said its suspension is designed to reassure the public that StarLink corn won't get into the food supply in the future. "It isn't in our interest to sell corn seed if it is causing confusion," said Aventis spokesman Rick Rountree. Stephen L. Johnson, EPA deputy assistant administrator for pesticides, called Aventis' decision "a prudent action that we fully support."

It is unclear whether Aventis can prevent the corn from entering the food chain. StarLink is grown on 315,000 acres on farms scattered from Texas to Illinois.

Federal regulators aren't yet sure how the taco-shell debacle happened in the first place. Some food industry officials suspect airborne pollen from a StarLink field might have blown into fields of conventional corn being grown under contract for the Azteca Milling facility in Plainview, Texas, which makes corn flour for Kraft's taco shell.

The EPA requires that farmers plant StarLink seed at least 660 feet from other cornfields to prevent cross-pollination. Organic farmers, who have turned their backs on biotechnology, have complained that such buffers aren't sufficient to protect their fields from pollen drift.

It is also possible that a farmer or grain handler accidentally mixed a shipment of StarLink corn with grain destined for Azteca Milling. Both types of corn look so similar that chemical analysis is needed to tell them apart.

The StarLink plant uses a gene transplanted from a common soil organism, Bacillus thuringiensis, to make a protein toxic to certain insects. StarLink's gene expresses a slightly different protein from the sort made by the other types of insect-resistant corn already approved for use in food. Of the 40 or so genetically modified crops on the market, StarLink corn is the only one that isn't approved for human consumption.

The taco-shell incident revealed that the food industry lacks a quick and reliable way to screen its products for the presence of genetically modified organisms.

It took Kraft several days to determine whether StarLink corn is in its taco shells, and some biotechnology officials still doubt Kraft's results. Some food companies testing their products in wake of the Kraft recall complain privately that the same sample can test positive and negative for StarLink, depending on what method is used.

Capitalizing on the confusion, Strategic Diagnostics Inc., a Newark, Del., testing firm, began selling the first rapid-field test for StarLink only yesterday. Arthur A. Koch Jr., Strategic Diagnostics chief operating officer, said the company is so swamped with orders from grain handlers it is rationing the kits. The field test is designed to detect the StarLink protein in raw corn, which means it doesn't work on processed foods such as taco shells.

The recall was ammunition for antibiotechnology groups in a Senate health committee hearing Tuesday. Groups such as Friends of the Earth want far tighter regulation of the young science by the Food and Drug Administration, among other agencies. They're lobbying for mandatory labels on food products that contain genetically modified organisms, an idea most big food companies oppose.

"It is not lost on consumers that the problem was discovered not by the FDA or EPA, but by Friends of the Earth," Consumers Union researcher Michael Hansen told the committee.

-- Sara Leuck in Washington contributed to this article.

Write to Scott Kilman at scot.kilman@wsj.com

** NOTICE: In accordance with Title 17 U.S.C. Section 107, this material is distributed for research and educational purposes only. **