A high dose immunization of mice with gamma-irradiated allogeneic spleen cells has been shown to induce, in a recipient spleen, specific suppressor T-cells, resistant to mitomycin C, which are capable of inhibiting DNA synthesis and, to a lesser degree, the generation of killer cells in the mixed lymphocyte culture (MLC). The maximum suppressor activity is reached on days 3-6 after immunization. Both reactions are blocked mostly in those stimulator cells which bear H-2 antigens used for immunization. In contrast, DNA synthesis is inhibited only slightly, if at all, when it is stimulated in MLC by third-party cells, even if these are added to the culture as a mixture with correspoding stimulators. Unlike X-irradiated allogeneic cells, the untreated ones induce a mixture of suppressors, T-cells and macrophages, with a considerable non-specific suppression. Untreated syngenic lymphoid cells induce less active non-specific suppressors with properties of macrophages.