SummaryMononuclear cells cytotoxic to methylcholanthrene‐induced fibrosarcoma cells were detected in the peritoneal cavities of normal rats by means of 16 h51Cr release and 48 h125IUdR release assays. Carriage of a tumour which induces concomitant immunity, or intraperitoneal injections ofCorynebacterium parvumor proteose‐peptone, led to increases in mononuclear cell numbers. Injections ofC. parvumand proteose‐peptone led to increases in cytotoxicity in51Cr release assays. Carriage of an immunogenic tumour led to an increase in cytotoxicity to homologous tumour cells in the125IUdR release assay. Cells were separated by sedimentation velocity, metrizamide density gradients and adherence to glass or plastic. Small cells were more active than large cells in51Cr release assays. Larger cells tended to be more active in125IUdR release assays. Generally, low density cells were more active in both assays, except that high density cells induced byC. parvumwere most active in125IUdR release. Adherent cells fromC. parvum‐injected rats were more active than non‐adherent cells in both assays. On the basis of differential stimulation and separation by sedimentation velocity it is suggested that two cell types, possibly NK cells and macrophages, are operative.125IUdR‐labelled fibrosarcoma cells were lysed after intraperitoneal injection into normal and concomitantly immune rats, more rapidly and extensively in the latter case. These cytotoxic cells may be important in both natural and acquired resistance of rats to tumour growth.