The immunomodulatory effects of both acute and chronic alcohol use/treatment have been presumed, because chronic alcoholism increases susceptibility to bacterial infections such as pneumonia and tuberculosis. In the present study, the effects of ethanol on phagocytosis of mouse peritoneal macrophages were examined by means of fluorescent particles and flow cytometry. Phagocytosis of latex beads, zymosan (Saccharomyces cerevisiae, S. cerevisiae) and Staphylococcus aureus (S. aureus) was suppressed by ethanol in a dose-dependent manner. The results suggest that the reduced function of macrophages, at least in part, accounts for an increased susceptibility of alcoholic patients to infection.