Purified cytoplasmic granules from cytotoxic rat large granular lymphocytes (LGL) tumors were cytolytic to erythrocytes, splenocytes, and a number of different lymphoid tumor cells. Granule concentrations of approximately 1 microgram/ml granule protein were adequate to lyse 100% of the erythrocytes, while the nucleated cells required up to 100 micrograms/ml granule protein to achieve complete lysis. Cytoplasmic granules purified from noncytotoxic lymphoid cells did not contain detectable cytolytic activity; purified granules from rat mast cells and rat liver lysosomes likewise failed to display cytolytic activity. However, granules prepared from normal rat peripheral blood LGL were cytolytic. Granule-mediated lysis of erythrocytes and nucleated cells was complete within 3 min at room temperature. The lytic activity required calcium at concentrations of 10(-4)-10(-2) M; magnesium or barium failed to replace calcium, while strontium could replace calcium at 10(-3)-10(-2) M when nucleated cells were the target. Exposure of LGL tumor granules to calcium before the addition of target cells resulted in an inactivation of granule cytolytic activity over the course of 20 min at room temperature. Granule cytolytic activity was heat and Pronase sensitive, and could be solubilized by 2 M salt. Examination of granules exposed to calcium in the electron microscope using negative staining showed that calcium treatment of granules results in the formation of ring-shaped structures previously described to be associated with LGL-mediated cytotoxicity. These results provide support for the hypothesis that the cytotoxic processes mediated by LGL are a secretory event characterized by the release of cytolytic material from the cytoplasmic granules after triggering by a surface receptor. The results further suggest that the ring structures visible in the electron microscope are associated with the lytic event.