Immunization of C311/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.hybrid cell; MM antigen; CTL Numerous investigations have revealed tumor cells to bear tumor specific transplantation antigens (TSTA) and, in general, that they elicit a cell-mediated anti-tumor response. For example, many tumor cells derived from spontaneous mammary carcinomas in C3H/He mice have a surface TSTA, named the MM antigen, which has never been detected in normal tissues or primary carcinoma cells (Nishioka et al. 1969a, b;Takeuchi et al. 1969;Irie 1971). It was found that the MM antigen on MM tumor cells is capable of inducing a predominantly humoral response in syngeneic C3H/He mice, while scarcely inducing any cellular response. Nishioka and his colleague first described that C311/He mice which had been inoculated with a mixture of MM2 ascitic mammary tumor cells and rabbit anti-MM2 antiserum intraperitoneally (i.p.), acquired resistance to subsequent MM2 tumor cell challenge, and that the sera from these mice contained tumor specific antibodies capable of protecting the hosts against MM2 tumor growth ( Nishioka et al. 1969a;Takeuchi et al. 1969).