The cytoplasmic granules purified from rat large granular lymphocyte tumors with NK activity have been studied to determine their role in cytotoxic and other functions of these lymphocytes. Biochemical purification of the cytolysin from the granules shows that it is a single peptide chain of about 65 kd. When erythrocytes are lysed with cytolysin, this molecule forms a large pore-like complex in the membrane which has been purified. Cytolysin activity was shown to be blocked by a variety of lipid analogues, in agreement with a membrane insertion mechanism. Anti-idiotype antibodies raised against the phosphorlycholine binding myeloma protein TEPC 15 were shown to bind to purified cytolysin and block its lytic activity, suggesting a choline recognition by cytolysin. The biological activities of non-cytolysin components of the granules were also studied. It was shown that a DNase activity is associated with LGL granules in the Percoll gradient. This activity was not part of the cytolysin. The DNAse activity has a neutral pH optimum and is found in much lesser amounts in non-cytotoxic lymphocytes. Collaborative studies with other labs have led to identification of other granule components, including a factor which is chemotactic to large granular lymphocytes and other cells, and chromogranin, an antigen previously known only in neuro-endocrine cell granules.