USDA officials said the animal could "very likely" turn out to be healthy. They refused to identify if the suspect animal was a cow or a steer, its age, location or any other information.

"Because the test may very well turn out to be negative, we're not going to disclose this information at this time," USDA's chief veterinarian, John Clifford, told reporters.

"This is not at all unexpected. Screening tests are designed to be extremely sensitive," Clifford said. A screening test "is very likely to have negative samples that show inconclusive (results)," he added.

The USDA's animal health laboratory in Ames, Iowa, will retest the animal's brain samples using more sophisticated immunohistochemistry tests. Final results will take four to seven days, Clifford said.

USDA officials said there was no risk to the public health because meat from the animal did not enter the human food supply. "This animal did not enter the human food chain nor the feed chain," Clifford said.

The inconclusive test result was detected late Friday afternoon as part of the USDA's stepped up testing of American cattle for the deadly disease.

Last year, the USDA tested about 20,000 cattle for mad cow disease. The department plans to test 220,000 by the end of 2005 using its new rapid tests.

Industry officials expressed concern about the impact the inconclusive test will have on cattle markets on Monday. "I hope we keep calm about this thing," said Tom Cook, president of the National Renderers Association.

U.S. meat groups have urged the USDA to refrain from publishing inconclusive test results because of the affect the news would have on the markets.

The USDA said publishing this information was the best way to ensure transparency on market-sensitive news. The first case of mad cow disease in the United States was found last December in a dairy cow in Washington state. The animal is believed to have been infected by eating contaminated livestock feed soon after it was born in Canada.

More than 130 people in Europe have died from a human form of mad cow disease, which is also known as bovine spongiform encephalopathy (BSE).

Until a few years ago, it was standard practice to recycle slaughterhouse leftovers such as brains, spinal cords, spleens and protein-rich bits into feed for cattle. Today, that material is banned from cattle livestock and the human food supply.

The December 2003 diagnosis in Washington state prompted virtually all U.S. trading partners to halt their purchases of American beef as a safety precaution.

Clifford said he did not expect Friday's uncertain test results to impact those countries that have resumed shipments of U.S. beef. "We don't think at this point in time this would affect trade with our trading partners," he said. "It's very likely this animal could be negative (for mad cow disease)."

U.S. trade officials were scheduled to meet again soon with Japan, formerly the biggest buyer of American beef, as part of a continuing U.S. push to get Tokyo to resume its purchases. Japan last year bought $1.4 billion worth of U.S. beef products.

U.S. consumer groups this week pressed the Food and Drug Administration to issue its long-delayed mad cow regulations that would prohibit certain cattle parts in food, cosmetics and pharmaceuticals.

The FDA announced its new rules in January but has yet to issue the details.