The fast-food chain commits to sourcing chicken raised without the use of some antibiotics.

OAK BROOK, ILL. — McDonald’s USA said it will only source chicken raised without antibiotics that are important to human medicine and will offer customers milk from cows that are not treated with recombinant bovine somatotropin (r.B.S.T.).

“Our customers want food that they feel great about eating — all the way from the farm to the restaurant — and these moves take a step toward better delivering on those expectations,” said Mike Andres, president of McDonald’s U.S.

The company said it will implement its antibiotics policy over the next two years, and emphasized the antibiotics it will continue to use will be those not critical to human medicines. Poultry farmers that supply chicken to McDonald’s will continue to use ionophores, a type of antibiotic not used by people.

“McDonald’s believes that any animals that become ill deserve appropriate veterinary care and our suppliers will continue to treat poultry with prescribed antibiotics, and then they will no longer be included in our food supply,” said Marion Gross, senior vice-president of McDonald's North America supply chain.

The fast-food chain also will offer jugs of low-fat white milk and fat-free chocolate milk from cows that are not treated with r.B.S.T.

“While no significant difference has been shown between milk derived from r.B.S.T.-treated and non-r.B.S.T.-treated cows, we understand this is something that is important to our customers,” Ms. Gross said.

Mr. Andres added that these may not be the only efforts the company makes to improve the perceived quality of McDonald’s menu items.

“We will continue to look at our food and menu to deliver the kind of great tasting and quality choices that our customers trust and enjoy,” he said.