Very recent evidence indicates that, in addition to its already known analgesic, anti-inflammatory, antipyretic, and platelet-aggregation-inhibiting properties, acetylsalicylic acid (ASA) exerts a positive influence on the immune system. Therefore, its action on murine peritoneal macrophages was investigated in the present study. Various ASA concentrations (50-200 micrograms/ml) failed to have any stimulating or inhibiting effect on antibody-dependent cellular cytotoxicity (ADCC) and on the binding capacity of macrophages from various levels of activation against SW 707 tumor cells. Furthermore, no time dependence of the stimulation was observed over a period of up to 48 hrs. These results suggest that the reported positive effect of ASA on the immune system, manifested by increased production of interleukins and interferon, is due to an interaction of macrophages and lymphocytes and not to a direct increase in macrophage activity.