Studies were designed to investigate whether the suppressor cell systems that regulate the humoral and cell-mediated immune responses belong to the same subsets of T cells or different subsets. Mitogen-activated suppressor cells were simultaneously assayed for their ability to inhibit (a) pokeweed mitogen-induced generation of plasma cells, (b) blastogenic response of lymphocytes to allogeneic cells, and (c) generation of killer cells in the cell-mediated lymphocytotoxicity assay. We found that suppressor cells that inhibited the generation of plasma cells were activated by concanavalin A (Con A) and were both radiation and prednisone sensitive. Suppressors that inhibited the blastogenic response in lymphocytes to allogenic cells were also activated by Con A but differed in that they were both radiation and prednisone resistant. In contrast, suppressors that inhibited the generation of the killer cells were activated with phytohemagglutinin and not Con A. These suppressors were prednisone and radiation resistant. These observations cannot be explained by differences at the pro-suppressor or suppressor activator levels as both T cell subsets are radiosensitive. Alternatively, heterogeneity of suppressor cell systems may explain these differences.