The cytotoxic cells from the spleens of normal rats, which lyse Gross-virus-induced lymphoma target cells in a short-term 51Cr release test, are predominantly small to medium-sized cells sedimenting at 4-5 mm/h as shown by velocity sedimentation analysis. Their cytotoxic activity is relatively resistant to gamma-radiation, 50% surviving 1,000 rads and 30% remaining after 5,000 rads, and to heat, since 20 min incubation at 48-51 degrees C is required for its abolition. In these properties the natural killer (NK) cells are very similar to the cytotoxic T cells from tumour-immune rats, and they share in addition a requirement for Ca++ ions for cytolysis. They differ in that they are non-T cells as defined by their resistance to anti-T-cell antiserum and complement, and by their presence in T-cell-deprived rats. They lack detectable surface Ig, Fc receptors and phagocytic or adherence properties and belong therefore to that small proportion of lymphoid cells lacking the surface markers of T or B lymphocytes, as do the comparable NK cells of mice. Cytotoxicity appears to involve an autonomous, papain-sensitive recognition structure on the surface of NK cells, rather than acquired cytophilic antibody operating through an antibody-dependent mechanism.