Peritoneal exudate cells (PEC) harvested from mice after i.p. injection of squalene-treated cell wall skeleton of Nocardia rubra (N. rubra-CWS) demonstrated vigorous cytolytic activity in vitro toward tumor target cells. Fractionation of these PEC by adherence to plastic dishes showed that the cytolytic activity in PEC was associated with an adherent phagocytic cell. Induction of the cytolytic adherent PEC required an optimal dose of 50 micrograms N. rubra-CWS and i.p. injection. Cytolytic activity of N. rubra-CWS-induced adherent PEC was maximal after 5 days and fell steadily thereafter. Susceptible tumor targets included cells syngeneic, allogeneic, and xenogeneic to the effector cell source. In contrast, nonneoplastic xenogeneic cells were not affected by N. rubra-CWS-induced adherent PEC. The effector cells were not found in the spleen or peripheral lymph nodes. In addition, the cytolytic activity of N. rubra-CWS-induced adherent PEC was completely inhibited by treatment with antimacrophage serum and complement or carrageenan. Treatment with monoclonal anti-Thy 1.2 antibody and complement, however, did not affect the cytolytic activity of the adherent PEC. These features make it likely that N. rubra-CWS-induced cytolytic effector cells are macrophages.