Many of the reactions of cellular immunity are mediated by soluble lymphocyte-derived factors (lymphokines). One important category of lymphokine action involves effects on cell motility. These effects have been described mainly with respect to inflammatory cells. In this report, we describe the ability of a lymphocyte product to inhibit the migration of endothelial cells in a system in vitro. The responsible factor is distinct from a previously described mediator that inhibits the migration of tumor cells. The ability of lymphocytes to influence the migration properties of endothelial cells is consistent with data of others showing a relation between the immune system and processes involving neovascularization.