The cytotoxic activity in peritoneal exudates harvested from C57BL/6 mice 4 to 6 days after they had received viable Bacillus Calmette-Guérin (BCG) organisms i.p. was associated with a nonadherent, nonphagocytic cell. The cytotoxic cell lacked demonstrable surface immunoglobulin and Thy 1 antigen and bore no readily detectable Fc receptors. Lytic activity was labile at 37°C and was diminished after trypsinization of the effector cells. Preincubating effector cells with immune complexes was without effect on lytic expression. These features make it likely that the cytolytic activity was associated with “natural killer” (NK) cells, previously described in unimmunized mouse spleens and mesenteric lymph nodes. Whether BCG induced “activation” of resident NK cells, the de novo production of NK cells, or “homing” into the peritoneum of cells normally resident in other lymphoid tissue is not known.