The company, which has spent tens of millions of dollars since June on an advertising and labeling campaign to distinguish its product from competitors, is scrambling to salvage the label.

The decision, conveyed in a previously nonpublic letter dated Nov. 6, comes after the Springdale, Ark., company received approval by the USDA in May to label its products as "raised without antibiotics," a coveted term in the industry that companies are seeking as a way to appeal to consumers concerned about the use of antibiotics in animals.

The labeling was a coup for Tyson, and the following month the company announced with fanfare that it would become the "first major poultry company to offer fresh chicken raised without antibiotics on a large-scale basis." At the time the label was approved, consumer-protection groups lauded Tyson's decision and urged other major poultry producers to follow suit.

Now the USDA says it made a mistake in approving the label. Daniel Engeljohn, an official at the USDA's office that oversees labeling decisions, said the department is trying to "make sure labels are truthful and not misleading" and "our consistent position is, if you're going to make a raised-without-antibiotics claim, that there will be no antibiotics that were included in the feed during the life of the animal."

A USDA spokeswoman said Tyson's "labels should not bear a no-antibiotic claim" and the Food Safety and Inspection Service of the USDA "mistakenly...approved the labels that were contradictory to this policy."

Tyson disagrees with the decision, and for the past few weeks has been in discussions with the USDA trying to "convince them that we are right and that they made a mistake," said Gary Mickelson, spokesman for Tyson. He said the company intends to have the issue resolved before Thanksgiving.

At the heart of the USDA's decision lies a type of animal medication called ionophores, which are commonly added to poultry feed to help prevent an intestinal colonization by coccidia, a single-celled organism that can lead to lower body weight or death in poultry, causing economic loss to producers.

Tyson said the product isn't technically an antibiotic, but instead is an antimicrobial. In the USDA's letter, however, the agency said "it is longstanding FSIS policy that ionophores are antibiotics."

Tyson also said the USDA approved the label earlier in May with full knowledge the company was using ionophores in its poultry feed. However, the USDA said it was "recently drawn to the attention of senior officials at the Food Safety Inspection Service" that Tyson's poultry-feed formulations contained ionophores.

The company has 45 days to remove the labels from its products or stop using ionophores. The agency also gave the company the option of petitioning the USDA to initiate a public-comment period on whether ionophores could be used in meat and poultry products labeled as "raised without antibiotics." Also, the agency gave Tyson the option of submitting a new label application with new documentation.

Tyson is in the process of submitting a new label application that would ask the USDA to allow the company to modify its label to explain that the medication isn't an antibiotic that presents the possibility of creating antibiotic resistance in humans.

Tyson said that because ionophores are metabolized by the bird before the meat reaches the dinner table, consumers don't need to worry about consuming meat raised on ionophores that could lead to human antibiotic resistance.

Margaret Mellon, director of the food and environment program for the Union of Concerned Scientists, a health and environmental organization in Washington, said she is "mystified" and "troubled" by the USDA's decision to rescind Tyson's label and to classify ionophores as an antibiotic.

Ms. Mellon, who works with a group called Keep Antibiotics Working, a coalition of consumer health groups, is opposed to the agency's classification because ionophores aren't used in human medicine. As a result, it is unlikely they could develop disease resistance that could lead to an undercutting of the efficacy of drugs in humans.

Also, she said ionophores don't target bacteria like antibiotics do. Instead, they target the single-celled organisms that can be found in livestock and poultry.

Tyson's research showed that 90% of consumers said they think it is important to eat chicken raised without antibiotics.

"Despite this process that we're going through, we remain committed to the raised-without-antibiotic program, and we fully intend to proceed with it," Mr. Mickelson said.