The migration of lymphocytes from several sources and several species was assessed in modified Boyden chambers. The migratory ability of resident thymocytes from cats, guinea pigs and rats is generally comparable. In rats a non-adherent T-cell population from lymph nodes and from the ductus thoracicus showed a higher spontaneous mobility than thymocytes. The migration of all tested lymphocytes was significantly stimulated by 5-HT and bradykinin in a dose-dependent manner. Lymphokine preparations with a MIF-activity less than 0.8 increased the migration of lymphocytes, too. It could be suggested that 5-HT and bradykinin together with lymphokines play a role in the movement of lymphocytes.