We have demonstrated that Rowett and NIH strains of random-bred athymic and euthymic rats exhibited significant levels of natural killer (NK) cell cytotoxicity against various murine and human tumor cell lines. Rats of both strains displayed high NK cell cytotoxicity in peritoneal exudate, peripheral blood and spleen, and lower activities in lung and liver. No activity was detected in bone marrow. In general, athymic rats manifested higher levels of NK cell cytotoxicity and higher tumor-binding potential than euthymic animals. NK cell cytotoxicity was subject to late maturation and did not decline with age. Cytotoxic cells were not removed by carbonyl iron treatment and resided in both adherent and nonadherent fraction after nylon-wool filtration, with the majority of the cytotoxicity in the latter fraction. Percoll gradient separation studies indicated that, in general, the cytotoxic cells were of large granular lymphocyte morphology and of low density. Evaluation of tumor-binding capacity and cytotoxic profile of large granular lymphocytes has shown that the majority of these cells exhibited cytotoxic potential, but only a minority of them displayed tumor-binding capacity.