An animal suspected of having mad cow disease based on a new rapid screening test has turned out to be uninfected, the Agriculture Department said yesterday.

USDA chief veterinarian John Clifford said more sophisticated testing conducted at the National Veterinary Services Laboratories in Ames, Iowa, showed that the cow was free of the disease.

The government has said that the new rapid testing is highly sensitive and known to produce false positive results for mad cow disease, known formally as bovine spongiform encephalopathy. Clifford said the USDA intends to continue announcing initial "inconclusive" results from the rapid screening to be "transparent" and open about its mad cow findings.

"The USDA remains confident in the safety of America's food supply," Clifford said after the announcement.

The first "inconclusive" result for mad cow disease was announced by USDA on Friday, and another was made public on Tuesday. Tissues from the carcass of the second animal were also sent to the national laboratory, and results from the sophisticated immunohistochemistry test are expected in four to seven days.

Announcement of the "inconclusive" mad cow tests caused the price of cattle to drop sharply yesterday at the Chicago Mercantile Exchange. The announcement that the first suspect animal was not infected came after the market closed.

The USDA began an expanded rapid testing program for mad cow disease in early June and plans to test more than 200,000 animals in the next 18 months. USDA officials have said the effort, initiated after the nation's first case of mad cow disease was detected in December, is intended to determine whether there is more of the disease in the American cattle herd.

Clifford said that because the first animal with an "inconclusive" rapid test turned out to not have mad cow disease, no further information will be released on its location. The agency will make public the location of any animals confirmed to have the disease, he said.

Most of the animals tested under the new rapid screening program are considered "high-risk" cattle that died for unknown reasons, are unable to walk or show signs of central nervous system disorders. Mad cow disease destroys the brains of infected animals and, in rare cases overseas, has been transmitted to people who ate meat from the diseased cattle.

The National Beef Cattlemen's Association said yesterday that the beef industry is not surprised by the recent findings.

"As part of USDA's expanded BSE surveillance program, a rapid screening test is used as the first step in a two-part testing process," the association's president, Jan Lyons, said in a written statement. "Because the rapid tests are sensitive, they are subject to occasional inconclusive results that later prove to be negative. It is a little like going through the airport metal detector."

The USDA has said that while many false positives are anticipated with the new tests, it also expects to find additional cases of mad cow disease.