When effector lymphocytes were reacted with cultured human tumors, the total cytotoxic reaction could be divided into selective and nonselective components. The nonselective part of the reaction was due to a cell type called N cells. Fractionation of effector suspension indicated that N cells were neither T nor B cells. like B cells, N cells did not form rosettes with sheep erythrocytes; they were retained by columns coated with Ig or antiserum to Ig and died preferentially when stored at an ambient temperature. However, N cells were differentiated from B cells by their inability to form complement-receptor rosettes and by their survival when incubated at 30° C. The effect of the nonselective cytotoxic cell must be differentiated from selective activity in studies of specificity for cell-mediated cytotoxicity.