Immunization of C3H/He mice with L-FM3A#2 hybrid cells, made by fusion of ascitic mammary carcinoma FM3A#2 cells with 8-azaguanine resistant LAG cells, both of C3H/He mouse origin, resulted in spleen T cell-dependent resistance to the parental FM3A/R cells. These spleen T cells, purified by passing through a nylon fiber column, could be demonstrated to have Thy-1.2 and Lyt-2.1 antigens, and not L3/T4 antigens. After immunizing with irradiated FM3A/R cells, cytotoxic cells other than cytotoxic T lymphocytes (CTL) appeared, these presumably being nonphagocytic macrophages or polymorphonuclear cells. In this case, anti MM antiserum was generated at an earlier stage than when mice were immunized with the L-FM3A#2 cells. The cytotoxic mechanism is discussed as to the significance of the surface antigen.