ASHINGTON, Dec. 23 ó A sick cow slaughtered about two weeks ago near Yakima, Wash., has tested positive for mad cow disease in early laboratory results, the first such case in the United States, the secretary of agriculture said on Tuesday.

Agriculture officials are likely to announce as early as Wednesday a voluntary recall on beef they hope to trace to the plants where the cow was slaughtered and processed, said Dr. Elsa Murano, the under secretary for food safety.

"We are considering if we need to take that step, but it's likely to happen," Dr. Murano said in an interview.

Federal officials did not say where the meat is now, but the agriculture secretary, Ann M. Veneman, said that the meat supply was safe because of precautions taken over the last decade to keep the nerve tissue of slaughtered beef out of the food supply. Only the brain, spinal cord and related parts can spread the disease to humans, Ms. Veneman said, and she added that she intended to serve beef to her family at Christmas.

"This finding, while unfortunate, does not pose any kind of significant risk to the human food chain," she said at a news briefing here tonight.

While agriculture officials urged the public not to overreact to the discovery, Dr. W. Ron DeHaven, the chief veterinary officer for the Agriculture Department, said: "This is certainly a big concern. We now have evidence of a disease that we didn't have before in the U.S."

Agriculture officials and leaders of the beef industry were particularly concerned about the impact on domestic sales and beef exports. When a single case of mad cow disease, known formally as bovine spongiform encephalopathy, was found in Alberta, Canada, in May, a number of countries, including the United States, banned the import of Canadian beef. The ban has been eased somewhat, and beef imports, of boneless cuts and from cattle younger than 30 months, have resumed.

No cases have turned up in people.

According to federal officials, the cow in the Washington case, a Holstein, was traced back to a farm in Mabton, about 40 miles southeast of Yakima. The farm has been quarantined, Dr. Veneman said.

The sample was taken on Dec. 9, the same day the cow was slaughtered. Inspectors took a sample because the cow was a "downer animal," which Ms. Veneman said meant "non-ambulatory."

Nerve tissue from the cow was tested at a government laboratory in Ames, Iowa, establishing a "presumptive" diagnosis, she said, and a military jet is flying a sample to a laboratory in England for a definitive diagnosis. No result is expected for several days, but the government was proceeding as if the finding was conclusive, she said.

The development is likely to be a serious blow for ranchers, feed-lot operators and slaughterhouses. Shortly after the announcement, Japan and South Korea announced that they were banning imports of American beef. About 10 percent of American beef production is exported, industry officials say.

McDonald's and Wal-Mart Stores quickly said they did not believe they had received meat from the animal.

And almost as soon as Ms. Veneman finished her news conference, officials of the National Cattlemen's Beef Association began a conference call to seek to reassure consumers. Terry Stokes, the chief executive officer of the group, referred to a "triple firewall" to prevent the introduction or spread of the disease.

Mr. Stokes said these safeguards consisted of testing animals that arrive at slaughterhouses unable to walk, forbidding imports of cattle and bovine products from countries where the disease is present, banning as food for cows material derived from cows. That is meant to prevent the transfer of aberrant proteins, called prions, which are believed to cause the disease.

Investigators are still trying to determine how and when the cow was processed.

"First we have to determine where the stuff went. That will determine how big the recall is," with officials hoping to recall any Washington beef that may have become mixed with and contaminated by the diseased cow, Dr. Murano said.

Dr. Murano said it was possible that the contaminated beef had already been distributed and eaten, but she said that even in that case she did not believe it posed a risk to consumers because the processed parts did not include the tissue that has been shown to carry the disease. Or the beef could have been frozen "and it may all be sitting in a warehouse somewhere," she said.

Dr. Murano said she expected the recall to be a "Class 2," the middle grade in the three-tiered system the U.S.D.A. uses to rank the severity of the health risk. "This is a voluntary thing out of an abundance of caution," she said.

Despite the evident failure of the system to prevent the case in Washington, Mr. Stokes said that consumers should have confidence in the food supply, because there is no evidence that the disease is transmissible through muslcle meat. Such a reassurance is critical since Agriculture Department officials said that meat from the infected animal, but not tissue from its central nervous system, had been sent to at least two other processing plants.

Critics say that the safeguards are not perfect. Among the problems, they say, is that machines that strip meat scraps from carcasses can contaminate the meat with tissue from the nervous system. Critics also say that regulations to prevent contamination of cattle food with nerve tissue are unevenly enforced.

"We put a number of measures in place that we thought would substantially reduce our chance of seeing mad cow disease in this country, but clearly those methods fell short of perfect," said Dr. Fred Cohen, a professor of pharmacology at the University of California in San Francisco and a leading expert on ways to treat prion diseases.

Still, Dr. Cohen said the risk was low.

"One can derive a fair bit of comfort from statistics and epidemiology," he said. "Put the question into context. When there were 60,000 to 80,000 infected cows in the U.K., approximately 150 people out of 60 million developed the disease," he said. "One cow is not likely to be translated into any cases" in the United States, he said.

The disease makes brain tissue spongy and full of holes. Sheep, deer and elk can also get spongiform encephalopothies. The human form is called Creutzfeldt-Jakob disease, and kills about 250 Americans a year. In most cases the cause is unknown.

Agriculture inspectors at the slaughterhouse in Mabton learned of the possibility of the disease when they tested the meat of a cow that had been unable to walk, a symptom of mad cow disease. A fraction of all cows in slaughterhouses that cannot walk on their own are tested for the disease.

The possibility of an infected cow renewed calls to end the slaughter of animals that cannot walk. Wayne Pacelle, the vice president of the Humane Society of the United States, said that such animals are pushed by bulldozers or dragged by chains, and are a threat to the food supply.

He said that the Senate had approved an amendment to the Agriculture Department's appropriations bill for the current fiscal year that would have forbidden such slaughter, but that the House had narrowly defeated it and a conference committee had left it out of the current version. The bill is part of the omnibus appropriations bill that Congress will face in January.

Ms. Veneman said in the news conference that her department had tested 20,526 head of cattle for mad cow disease this year, triple the level of last year.

The diagnosis in Washington State came just a week after a federal appeals court in New York resuscitated a lawsuit brought by an animal rights group that believes the Agriculture Department has not done enough to protect consumers from mad cow disease.

The group, Farm Sanctuary, maintained in a 1998 lawsuit that the government's policy of allowing the slaughter of animals that cannot walk poses a significant health risk to consumers. A judge threw out the lawsuit, saying the danger was remote, but the Court of Appeals for the Second Circuit overturned that decision last week and revived the lawsuit.