The studies were performed on 20 conditioned cyclists and 19 untrained men. At rest absolute and percent number of neutrophils, eosinophils, and monocytes, neutrophil bactericidal activity, and blood plasma beta-glucuronidase, acid phosphatase, and lactic dehydrogenase (LDH) activities were similar in sportsmen and untrained men, while neutrophil adherence was lowered in sportsmen. Maximal physical exercise induced significant rises in absolute numbers of neutrophils and monocytes in both groups. In sportsmen, adherence of neutrophils and monocytes and neutrophil bactericidal activity significantly decreased under the influence of exercise, while neutrophil phagocytic activity did not change. On the other hand, in untrained men, maximal physical exercise did not induce significant changes in neutrophil and monocyte adherence and bactericidal activity of neutrophils, but their phagocytic activity increased. Blood plasma beta-glucuronidase, acid phosphatase, and LDH activities increased during exercise in both groups. The changes observed tended to normalize during 2-h recovery. The results obtained suggest that intensive physical exercise tends to depress nonspecific immunity, which may render sportsmen more susceptible to infections.