We analyze both the sources of U.S. states’ licensure requirements for international medical graduates (IMGs), and the effect of these regulations on migrant physicians’ choice of U.S. state in which to work over the period 1973-2010. Analysis of original data shows that states with self-financing state medical licensing boards, which can more easily be captured by incumbent physicians, have more stringent IMG licensure requirements. Additionally, we find that states that require IMGs to complete longer periods of supervised training receive fewer migrants. Our empirical results are robust to controls for states’ physician labor market. This research identifies an overlooked dimension of international economic integration: implicit barriers to the cross-national mobility of human capital, and the public policy implications of such barriers.