John S. Koch, 50, a partner in the Washington law firm of Covington & Burling who specialized in antitrust and trade association matters, died of a lung ailment April 28 at George Washington University Hospital.

Mr. Koch, who lived in Washington, joined Covington & Burling in 1959, when he graduated from the law school of Northwestern University and moved here. He became a partner in 1968. He was general counsel for United Way of America.

He was born in Chicago. In 1953, he graduated from Yale University, where he was managing editor of the Yale Daily News, the student newspaper. He spent the next year in Berlin writing a novel. He then returned to Chicago and was a reporter for the City News Bureau and the Chicago Daily News. He gave up journalism to enter law school at Northwestern. He was on the Law Review.

Mr. Koch was a director of the Friends of Tregaron, an organization that seeks to preserve the open spaces of the former Tregaron estate in the Cleveland Park section of the city. He also was a member of the Metropolitan and City Tavern clubs.

Survivors include his wife, the former Constance Chadwell, and four children, Anne Elizabeth, John S. Jr., Sarah Grace and Peter Chadwell, all of Washington; a brother, Robert A., of Port Washington, N.Y., and a sister, Caroline Koch of Chicago.