Spurred by growing fear that drugs or chemicals made in gene-altered plants will taint the food supply, the North American biotechnology industry is adopting a broad moratorium on planting certain types of crops in major food-producing regions.

The voluntary ban, which goes beyond any proposed government regulation, is designed to prevent the spread of exotic genes into field crops likely to be used for food or animal feed. Its most immediate impact will be to bar companies from planting certain types of gene-altered corn in the Midwest farm belt or from planting some types of the rape plant (from which canola oil is produced) on the Canadian prairie, but the ban could eventually apply to numerous crops and regions.

Michael J. Phillips, executive director for food and agriculture at the Biotechnology Industry Organization, outlined the new policy in an interview yesterday. The Washington organization, the trade group of the North American biotechnology industry, formally adopted the plan several days ago, after more than a year of intensive discussions. Word of it has been filtering out to interested groups, but the policy has not previously been disclosed to the public.

Though the policy is voluntary, a dozen companies in the United States and Canada that are trying to produce pharmaceuticals and industrial chemicals in plants have endorsed it, and newcomers would be likely to face strong industry pressure to go along. "If you are in the Midwest corn belt with a test plot today, you will not be there as a BIO member company in 2003," Phillips said.

The policy, he added, is designed to prevent a recurrence of the debacle that struck the food and biotechnology industries two years ago. Genetically engineered StarLink corn, approved only as animal feed, wound up in taco shells and other food products. No illness was convincingly attributed to the contamination, but recalling the tainted products cost companies hundreds of millions of dollars.

The altered genes in StarLink corn merely made the plant more resistant to insects. Biotech companies have far more ambitious plans: They say plants hold enormous promise as factories for producing drugs. Tests have already shown that human genes inserted into the plants can prompt them to make large quantities of medically useful proteins, which can then be refined and bottled like any other drug. Companies are preparing to test treatments made this way for herpes virus, respiratory disease and a host of other ailments. Useful industrial compounds could be made cheaply by the same techniques.

But there's growing fear, among environmentalists, food producers and even many biotechnology companies, that the exotic genes in these plants could spread to food crops on nearby farms as pollen is transferred by wind or insects. The industry fears that the ensuing public-relations disaster -- "your heart medicine in my cornflakes," in a common catchphrase -- would kill the technology.

"I think we can all agree that this industry cannot afford StarLink II," said Michael H. Pauly, executive director of biotechnology for Epicyte Pharmaceutical Inc. of San Diego, which is a year away from testing a herpes drug grown in corn. "One incident like that is unacceptable. It's going to require a certain standard of behavior from the entire industry."

A company in Texas is already making an industrial enzyme in corn. But no human drug made this way has come to market, and few companies have gotten past test plantings. For that reason, the policy's near-term impact should be limited, forcing companies to move test plots of gene-altered corn from the Midwest, where a handful were planted this season, to states like Arizona and Hawaii, where corn is not a significant food crop.

The bigger impact is likely to come over the next three to five years, as companies draw up plans for commercializing drugs now in the early stages of development. While the moratorium could steer projects to some states, others -- particularly the corn states of the Midwest -- are likely to lose small but valuable drug facilities that they had counted in their long-range economic plans. Already, midwestern secretaries of agriculture who know about the new policy have expressed reservations to BIO.

At the other end of the spectrum, many environmental groups are likely to find the voluntary action insufficient. A coalition of these groups, while acknowledging that the technology holds potential for human health, has called for producing gene-altered food plants only inside strictly segregated buildings, or making drugs only in plants never used as food.

"I'm sure the industry is feeling great about this policy, but I still think it's pretty weak," said Matt Rand, biotechnology campaign manager at the National Environmental Trust in Washington. He said the public should be especially concerned about a new technology when "even the industry lobby group recognizes that there's a problem."

The U.S. Food and Drug Administration and Department of Agriculture and their counterparts in Canada have been devising policies to manage the risk the crops will pose. Generally, the rules are aimed at separating the biotech crops from field crops enough to prevent gene transfer.

The BIO moratorium goes beyond these government requirements, but it does permit continued field tests in many states. It bans plantings only in regions where a particular crop is of considerable economic importance, as measured by the USDA, and is also prone to spreading its genes around. Corn and the canola plant are among the most promiscuous plants in this way.

To dodge the contamination problem, some biotech companies have already switched to more controllable plants, such as safflower, that are planted only in limited quantity in North America and that don't readily spread pollen.