Effect of neurotropin (NSP) was studied on the oxygen consumption and adherence capacity of polymorphonuclear leukocytes of peripheral blood. Its therapeutic doses were found not to affect the phagocytic and adherent ability of leukocytes of healthy adults. In vitro experiments demonstrated that the respiration of leukocytes ceases much earlier in the presence of NSP than in its absence in a dose-dependent fashion, although the initial velocity of oxygen consumption associated with phagocytosis was unchanged. The fact that respiration tended to become normal when dialyzed neurotropin was used suggested that some inhibitory factors, which are dialyzable, are present in this preparation.