Immunocompetent mouse thymus cell subsets (TH-2) cultured with allogeneic TH-2 cells, required a peripheral lymphoid cell in order to generate specifically cytotoxic T cells (CTL) in vitro. The helper cell is identified as a B cell, and can be supplied either in the mixture of target or responding peripheral cells, or in optimal ratios of 10:1 to 20:1 as nonproliferating cells syngeneic to the responder, when both responder and target are TH-2. Allogeneic helper cells are much less effective. Kinetic studies indicate that helper cell activity involves initial interaction with stimulator cell and is then required throughout the period of generation of cytotoxic lymphocytes. Addition of critical levels of certain IgG preparations, particularly IgG2A appears to support generation of CTL in TH-2/TH-2 cultures but is considerably less effective than intact B cells. Cytotoxicity generated under these conditions has the same genetic requirements for I-region histoincompatibility between stimulator and responder, and for K or D identity of stimulator and target cell, as has been reported for peripheral cell CTL.