Peritoneal exudate cells (PEC) from conventional donors displayed in vitro the cytotocix effects on the rat sarcoma cell lines LWI3K2, LW13K3, LW13K4 and RSL but they were not cytotoxic to fibroblasts derived from normal rats of the same strains. PEC were also cytotoxic to human urinary bladder carcinoma cells (T-24) grown in vitro. Cytotoxicity was detectable already at the effector to target cell ratio of 1 : 1, and the effector cells from syngeneic, allogeneic and xenogeneic donors were equally effective. The cytotoxic effect was apparently exerted by macrophages because the addition of lymphocytes from PEC donors did not change the target cells. The addition of serum from PEC donors to a mixture of target and effector cells protected the tumour cells from the destruction by macrophages. Cytotoxicity was blocked by serum from all PEC donors tested (syngeneic, allogeneic and xenogeneic with regard to the target cells) but not by foetal calf serum.