The phenotype of lymphocytes, obtained from mice immunized with allogeneic tumor cells, with the capacity to induce macrophage cytotoxicity was determined. Macrophage cytotoxicity was induced, either by incubating the macrophages with Macrophage Arming Factor (MAF) containing supernatants of cultures of sensitized lymphocytes and tumor cells (arming) or by incubating the macrophages directly with sensitized lymphocytes and tumor cells (activation). The MAF producing or activating capacity of the lymphocytes was not only "triggered" by the sensitizing tumor cells but also by normal cells and other tumor cells bearing the H-2 determinants of the sensitizing tumor cell. The capacity to render macrophages cytotoxic was not reduced after treatment of the lymphocytes with mitomycin-C or treatment with anti-murine Ig and complement. This capacity of the lymphocytes was abrogated after treatment with anti-T-cell serum or anti-Thy 1.2 serum and complement. After treatment with anti-Lyt 1 or anti-Lyt 2 serum and complement, the activating capacity was significantly reduced and the MAF producing capacity of the lymphocytes abrogated. Mixing the Lyt 1 depleted and Lyt 2 depleted lymphocytes or addition of normal lymphocytes to the Lyt 1 depleted or Lyt 2 depleted populations did not restore the MAF producing and activating capacities. This indicated that the lymphocytes inducing macrophage cytotoxicity in this allogeneic system are Lyt-1+2+ T-lymphocytes, which do not need to divide prior to perform their action.