This study has demonstrated the presence of large granular lymphocytes (LGL) in the rat and implicated a central role for these cells in mediating natural killer (NK) cell activity in this species. The LGL frequency from various organs was shown to be: peripheral blood = lung greater than spleen greater than peritoneal exudate greater than lymph node. Little or no LGL were found in the thymus or bone marrow. This pattern was similar to the distribution of NK activity, except for the lungs, which had no detectable activity. These studies have also shown that discontinuous Percoll density gradients can be used to highly enrich or deplete LGL from both spleen and peripheral blood. Enriched LGL populations were shown to bind selectively to NK-susceptible target cells and to demonstrate very high NK cell cytolysis. Since human NK activity has also been found to be clearly associated with LGL, the present findings indicate that rats may provide a very useful animal model for detailed studies of the ontogeny, regulation, and in vivo relevance of NK cells.