The present studies further characterize natural killer (NK) cells in the rat. The results demonstrate substantial differences in the cells which mediate NK activity (large granular lymphocytes (LGL)) and normal rat T cells. Studies on lymphokine-activated killer (LAK) cells indicate that the major precursor of LAK activity in the rat is an LGL. Analysis of the granules from LGL have led to the isolation of two cytolytic proteins (cytolysin and natural killer cytotoxic factor), at least two BLT-esterases, LGL specific chondroitin sulfate A glycosaminoglycans and proteoglycans, and an anti-fungal molecule which inhibits the growth of Cryptococcus neoformans. These results further characterize the functional nature of NK cells and suggest that LGL represent an entirely separate third lineage of lymphocytes, distinct from both T and B cells.