Peritoneal macrophages elicited in C3H/HJ mice by the i.p. injection of Corynebacterium parvum were cytotoxic to allogeneic virus-transformed fibroblasts in vitro. Cytotoxicity was demonstrated in a morphologic (plaque) assay, and quantitated by measuring macrophage-mediated inhibition of incorporation of 3H-thymidine by the target cells. The cytotoxic effect was well established by 6 hr of macrophage-fibroblast interaction, and was retained in cultures from which the supernatant was removed before the addition of 3H-thymidine. Cytotoxic activity of macrophages diminished rapidly after 22 hr of cultivation in vitro. Maximal cytotoxic effect could be prolonged by addition of C. parvum, 50 microgram/ml to macrophage monolayers preincubated in vitro for 22 hr. It could neither be retained nor regenerated when C. parvum was added to monolayers greater than 22-hr old. C. parvum-activated macrophages, grown under anaerobic conditions for 8 hr, retained the ability to phagocytize heat-killed Candida albicans and to exclude trypan blue dye. There was a small but significant reduction in the ability of macrophages to inhibit 3H-thymidine incorporation by target fibroblasts under anaerobic conditions. The cytotoxic effect of activated macrophages in air was not altered by the presence of catalase and was enhanced by enzymatically active superoxide dismutase. We conclude that the processes involved in macrophage-mediated cytotoxicity against allogeneic fibroblasts in this system are largely independent of oxygen.