Respondents are members of the Service Employees International Union, Local 32BJ (Union). Under the National Labor Relations Act, the Union is the exclusive bargaining representative of employees within the building-services industry in New York City, which includes building cleaners, porters, and doorpersons. The Union has exclusive authority to bargain on behalf of its members over their “rates of pay, wages, hours of employment, or other conditions of employment,” 29 U. S. C. §159(a), and engages in industry-wide collective bargaining with the Realty Advisory Board on Labor Relations, Inc. (RAB), a multiemployer bargaining association for the New York City real-estate industry. The agreement between the Union and the RAB is embodied in their Collective Bargaining Agreement for Contractors and Building Owners (CBA). The CBA requires union members to submit all claims of employment discrimination to binding arbitration under the CBA’s grievance and dispute resolution procedures.

Petitioner 14 Penn Plaza LLC is a member of the RAB. It owns and operates the New York City office building where respondents worked as night lobby watchmen and in other similar capacities. Respondents were directly employed by petitioner Temco Service Industries, Inc. (Temco), a maintenance service and cleaning contractor. After 14 Penn Plaza, with the Union’s consent, engaged a unionized security contractor affiliated with Temco to provide licensed security guards for the building, Temco reassigned respondents to jobs as porters and cleaners. Contending that these reassignments led to a loss in income, other damages, and were otherwise less desirable than their former positions, respondents asked the Union to file grievances alleging, among other things, that petitioners violated the CBA’s ban on workplace discrimination by reassigning respondents on the basis of their age in violation of Age Discrimination in Employment Act of 1967 (ADEA), 29 U. S. C. §621 et seq. The Union requested arbitration under the CBA, but after the initial hearing, withdrew the age-discrimination claims on the ground that its consent to the new security contract precluded it from objecting to respondents’ reassignments as discriminatory. Respondents then filed a complaint with the Equal Employment Opportunity Commission (EEOC) alleging that petitioners had violated their ADEA rights, and the EEOC issued each of them a right-to-sue notice. In the ensuing lawsuit, the District Court denied petitioners’ motion to compel arbitration of respondents’ age discrimination claims. The Second Circuit affirmed, holding that Alexander v. Gardner-Denver Co., 415 U. S. 36 , forbids enforcement of collective-bargaining provisions requiring arbitration of ADEA claims.