The effects of mast cell granules (MCGs) on macrophage-mediated lysis of P815 mastocytoma cells and nitric oxide (NO) production were studied. Murine peritoneal macrophages exhibited tumor cell killing and NO production only when activated with lipopolysaccharide (LPS) or interferon-gamma (IFN-gamma). Coincubation of macrophages with MCGs during LPS activation dose-dependently inhibited macrophage-mediated tumor cell lysis. The MCG effect was not due to inactivation or removal of LPS by MCG. The inhibitory effect was also not due to histamine or serotonin present in the MCGs. The granules were not toxic to macrophages or P815 mastocytoma cells. The effect of MCGs on macrophage-mediated tumor cell killing was evident whether MCGs were added before or after a 4-h exposure of macrophages to LPS. However, the inhibitory effect was not seen if MCGs were added after macrophages had been exposed to LPS for 24 h. To assess whether MCGs could inhibit a non-LPS trigger, MCGs were tested on macrophages activated with IFN-gamma. In these experiments, MCGs dose-dependently inhibited macrophage-mediated tumor cell killing induced by IFN-gamma, LPS, or IFN-gamma plus LPS. Furthermore, in parallel experiments, MCGs significantly inhibited macrophage NO production induced by LPS, IFN-gamma, or IFN-gamma plus LPS. Pretreatment of MCGs with diisopropylfluorophosphate, a serine protease inhibitor, only partially abrogated the effects of MCGs. The results demonstrate that MCGs inhibit both LPS- and IFN-gamma-induced macrophage killing of P815 cells and the inhibition is associated with the decrease of NO production.