Peritoneal macrophages from C56BL/6J mice, when activated by bacillus Calmette-Guérin, lysed syngeneic MCA-I sarcoma targets but not syngeneic embryo fibroblasts. Inflammatory macrophages elicited by concanavalin A (Con A) did not appreciably lyse either target. The activated macrophages secreted more neutral proteases into the extracellular compartment, both absolutely and relative to intracellular content, than did the Con A inflammatory macrophages. Bovine pancreatic trypsin inhibitor (BPTI) (750 KIU/ml) and diisopropylfluorophosphate (2 x 10(-3) M) inhibited cytolysis of neoplastic targets by the activated macrophages. The BPTI had to be present during the 48-hr macrophage-tumor cell interaction to reduce cytolysis; pretreatment of either the macrophages or the targets by the BPTI did not reduce cytolysis. The inhibitors, at the concentrations found to inhibit cytolysis, were not toxic to the macrophages as judged by morphology, by the ability of the macrophages to incorporate leucine into protein, and by the potential for cytolytic activation of the macrophages in vitro. It is suggested that neutral serine protease(s) secreted by activated macrophages participate in the cytolytic destruction of neoplastic cells.