PIP: This paper sets forth a procedure for calculating the annual efficiency gains from alternative changes in existing international immigration restrictions and evaluates the impact of wage rate changes on nonmigrating labor. Data on US gross national product (GNP)/capita across countries are used to infer differences in the marginal productivity both between countries and across major world trading areas. The method assumes that the worldwide labor supply is fixed, that full employment occurs in all regions, and that differences in labor's marginal product across regions arise because of barriers to inward mobility of labor in high-wage countries. When these barriers are removed, labor is assumed to be reallocated and efficiency gains occur. Results of the calculations suggest large gains from the removal of global immigration controls which, in most cases, exceed existing worldwide GNP generated in the presence of labor mobility restrictions. A large portion of the gain is accounted for by labor migration between the aggregated rich and poor countries. Over 40% of the total potential gain is realized when only 10% of the wage differential is eliminated, suggesting that small changes in global migration restrictions have large marginal effects. Wage rates increase in labor-losing regions and decline in labor-receiving regions, dramatizing the incentives for labor unions in high-wage countries to oppose liberalization of immigration restrictions. These results suggest large potential worldwide efficiency gains from a move toward an international labor market free of immigration controls. This issue may be far more important to the North-South debate than a focus on initiatives such as commodity price stabilization, relaxation of trade protection, or increased aid flows.