AbstractThe cellular requirements for the generation of cytotoxicity in mixed lymphocyte reaction (MLR) across an H‐2K mutant difference were analyzed. Treatment of C57BL/6 lymphocytes with either anti‐Lyt‐1 or anti‐Lyt‐2 serum and complement strongly reduced their capacity to mount a cytotoxic response against the B6.C‐H‐2bamutant. Almost complete reconstitution of the cytotoxic response occurred when a mixture of anti‐Lyt‐1 and anti‐Lyt‐2‐treated cells was allowed to respond. Once cytotoxicity was generated, only anti‐Lyt‐2 serum affected it. It is concluded that in the cytotoxic response to the ba mutant, Lyt‐1 helper cells collaborate with Lyt‐2,3 killer cell precursors in the generation of Lyt‐2,3 killer cells. Lyt‐1,2,3 cells play, if any, a minor role in the generation of cytotoxicity.By comparison, the capacity to generate killer cells in MLR against an H‐2K + IA difference was only affected by anti‐Lyt‐2 and not by anti‐Lyt‐1 serum, indicating that this response is relatively independent of he