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 multiple-state issues. Some previous literature has investigated whether states engage in free-riding 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 cross-state impacts (from the Multi-State 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). (JEL L40, H41, H77)