While there has been a considerable literature exploring determinants of antitrust enforcement in the United States, studies have been based either on aggregate federal enforcement data over time (exploring cyclical influences) or cross-industry studies, usually for a single year or aggregated over several years. What has never been investigated is the pattern of state-level antitrust. This is somewhat surprising, as this has been a major activity of many state Attorneys General. In this paper, we explain state antitrust enforcement across states and time (for a 15-year period), examining a number of economic and political determinants which have been proposed in the literature.