The Allied Chemical and Alkali Workers union (plaintiff) represented Pittsburgh Plate Glass Co.’s (defendant’s) hourly employees. In 1950, the parties agreed to a group health-insurance plan that included retirees. When Congress enacted Medicare in 1965, the company planned to cancel the health plan for retirees and substitute Medicare coverage. After the union objected, the company wrote each retiree, offering to pay Medicare premiums if the retiree withdrew from the group plan. The union filed unfair-labor-practice charges under the National Labor Relations Act (NLRA). The National Labor Relations Board (NLRB) found changing pensioner benefits subject to mandatory bargaining under the NLRA and issued a cease-and-desist order, but the appellate court reversed and refused to enforce it. The Supreme Court granted review.