But a positive finding could prompt USDA to do additional testing, delaying any announcement, according to a meat industry source who asked not to be identified.

In the meantime, USDA Undersecretary JB Penn said U.S. officials were talking with counterparts in major beef importing countries, including Mexico and Japan, trying to assure them that adequate steps had been taken to guard against the spread of mad cow disease. "All our trading partners have been informed," Penn told reporters.

Cattle futures trading at the Chicago Mercantile Exchange was volatile this week because of renewed mad cow concerns. June contracts closed 2.5 cents per pound lower at 87.1 cents.

"If it's negative, we could bounce back right away," said Jim Brooks, a livestock trader with brokerage R.J. O'Brien. "A lot of people were surprised we haven't seen an inconclusive before now. We could see one every two to three weeks."

The first case of mad cow disease was discovered in the United States last December in a Washington state cow that was imported from Canada.

In reaction, the USDA in March announced expanded testing for the animal brain wasting disease that has been linked to a human variant illness. The new test is more sensitive, raising the likelihood of false positive tests, according to USDA.

Friday's inconclusive test was the first under a program that will test more than 200,000 cattle over some 18 months.

Penn said that even if a second case of mad cow disease is diagnosed, that does not "change our risk profile." He added that Mexico, Canada and other importers of American beef should not erect any new trade barriers.

American exporters were worried, however.

"There is some concern within the (beef) industry on how Mexico is going to react" if the test confirms a second infected U.S. animal, said another U.S. industry source who asked not to be identified.

While the industry said Mexico likely would maintain imports of U.S. beef, "If you lose Mexico, you're taking a huge step backward," he added. Mexico is the second largest importer of U.S. beef. The premier importer, Japan, has banned all American beef since the Dec. 23 case in Washington state.

Mexico's agriculture ministry this week said it was maintaining import safeguards that allow only certain types of beef from the United States. But the head of the Mexican Meat Council said he feared a second case of mad cow disease in the U.S. could shut the border to beef trade.

Japanese officials were in Colorado this week for a previously planned three-day visit to beef processing and rendering plants. The delegation also plans to view animal feed facilities and laboratories for conducting mad cow tests.

USDA and the U.S. beef industry hoped Japan might start easing its total trade ban by the end of summer. It was still unclear whether a positive finding from the inconclusive test would hurt U.S. efforts.

In the meantime, the USDA refused to identify the suspect animal's age, location or even whether it is male or female.

Until the more exhaustive test results are in hand, the USDA "won't release any information" on the animal that produced the inconclusive test, said spokeswoman Julie Quick.

That drew an angry response from one U.S. cattle group that criticized the USDA's past handling of mad cow-related issues.

"As a result of providing too little information, the cattle market is acting quite volatile," said R-CALF United Stockgrowers of America Chief Executive Officer Bill Bullard. "At the very least, they (USDA) should have provided the origin of the animal if it was known."