Administration of C. parvum to both intact and thymectomized mice resulted in the appearance in the peritoneal exudate of cells which inhibited tumour growth in vitro. This effect was mediated by intact, viable adherent cells, which it seems reasonable to categorize as macrophages, and was contingent on contact between the effector and target cells. No co-operation was observed between lymph node cells from C. parvum treated mice and peritoneal exudate cells from normal mice.