WASHINGTON – A cow in Alabama has tested positive for mad cow disease, the Agriculture Department said yesterday, confirming the third American case of the brain-wasting ailment.

The cow did not enter the food supply for people or animals, officials said. The animal, unable to walk, was killed by a local veterinarian and buried on the farm.

“We remain very confident in the safety of U.S. beef,” the department’s chief veterinarian, John Clifford, said.

Authorities said the farm was under an informal quarantine but would not say where it was.

“We will not release this information at this time until we complete our investigation, and that could take a few days,” the Alabama agriculture commissioner, Ron Sparks, said.

Federal and state investigators are working to determine where the cow was born and raised and locate its herdmates and offspring. Mr. Sparks said there are no suspect animals on the farm.

The news came as the Bush administration worked to reassure Japan and other foreign customers of American beef. Japan halted American beef shipments in January after finding veal cuts with backbone – cuts that are eaten in America but not in Asia.

Japan was the top customer of American beef until the first American case of mad cow disease prompted a ban it had only recently lifted.

“We would not anticipate that this would impact our ongoing negotiations,” Mr. Clifford said. “Our product is safe. We’ve got a number of interlocking safeguards. And Japan themselves have had 20-plus cases of BSE.”

Mad cow disease is the common name for bovine spongiform encephalopathy, or BSE. The local vet examined the Alabama cow’s teeth and said the animal was older, “quite possibly upwards of 10 years of age,” Mr. Clifford said. Investigators are working to pinpoint the cow’s age, he said. The age of the cow is important because America put safeguards in place nine years ago to prevent the disease from spreading. America banned ground-up cattle remains from being added to cattle feed in 1997.