Effect of nicotine on chemiluminescence (CL) responses of human polymorphonuclear leukocytes (PMN) in vitro was investigated. Nicotine, ranging from 10(-5) to 5 X 10(-4)M, showed a concentration-dependent inhibition of the CL induced by 2.5 mg of opsonized zymosan. This inhibitory effect of nicotine (10(-4)M) on CL was not affected by atropine (10(-4)M), hexamethonium (10(-4)M) or acetyl beta-methylcholine (10(-4)M). These results indicate that nicotine directly acts to PMN with no relation of cholinergic affecting receptors. As to cytotoxic effects on PMN, assessed by the methods of trypan blue exclusion and of lactate dehydrogenase leakage from the cells, nicotine showed no cytotoxic effect on PMN in its range of 10(-5) to 5 X 10(-4)M. It is concluded that tobacco alkaloid nicotine could directly inhibit phagocytizing function of PMN. Furthermore, it is presumed that PMN circulating in the respiratory organs in tobacco smoking would be exposed to high concentrations of nicotine, and that the nicotine incorporated into the circulating system could suppress PMN functions resulting in harmful influences on the host defense mechanisms.