Recent research has documented a substantial role in antitrust enforcement by U.S. states. While many of the cases litigated involve small local firms, a nontrivial portion encompasses multiplestate issues. Some previous literature has investigated whether states engage in freeriding behavior in environmental regulation, and whether governments free ride on private decisions in provision of public goods. In this paper, we analyze a sample of antitrust cases involving crossstate impacts (from the MultiState Antitrust Database, provided by the National Association of Attorneys General) and explain the determinants of free riding (which we define as participating in a case, but not as a lead plaintiff).