Petitioner sued respondent unions, claiming that their lobbying, litigation, and other concerted activities violated federal labor law and antitrust law. After petitioner lost on or withdrew each of its claims, the National Labor Relations Board decided petitioner had violated federal labor law by prosecuting an unsuccessful suit with a retaliatory motive. The Court of Appeals affirmed. Because we find the Board *520 lacked authority to assess liability using this standard, we reverse and remand.

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