Spontaneous cell-mediated cytotoxicity has been shown to be augmented after therapeutic infarction of human renal carcinomas. The effector cell responsible for the enhanced cytotoxicity was characterized by fractionation by adherence to nylon-wool and serum-coated plastic surfaces, density, and reactivity with certain monoclonal antibodies. The effector cell was nonadherent, sedimented in the lighter fractions of a 40-53% discontinuous Percoll gradient, and was not affected by preincubation with OKT 3 antibody selectively blocking cytotoxic T lymphocytes. About 70% of effector cells expressed the HNK-1 and 50% the OKM-1 antigenic markers. Cytotoxicity was high against the NK-sensitive target K-562 and low against the relatively NK-resistant target cell Daudi. These data define the effector cell mediating the increased cell-mediated cytotoxicity induced by infarction of renal carcinomas as a natural killer cell.