Suppressor cells (SC) that nonspecifically inhibited lymphoproliferative (LP) responses were found after culturing peripheral blood mononuclear cells: with suppressor T-cell clones, in mixed lymphocyte cultures (MLC), and with recombinant interleukin 2 (IL-2), but were not found after culture in medium alone. A monoclonal anti-IL-2 receptor (R) antibody (MoAb), TU69, which blocked LP responses of IL 2-dependent T-cell lines, also blocked SC induction by T-cell clones, but completely failed to inhibit SC generation in MLC or with IL-2. This suggests that the IL-2R epitope defined by TU69 was not involved in SC induction in the latter systems. MoAb against HLA-DQ (TU22), -DR (TU34, SG157), -DP (B7/21), or DR and DP (TU43, 58), all of which were able to block stimulation of appropriately specific clones, did not block SC induction in any of the three systems studied. In contrast, the broadly reactive moAb TU39, which binds at least DR and DP but also has additional reactivity for determinants tentatively designated "DY," blocked SC induction by T-cell clones and in MLC. Finally, an anti-HLA class I MoAb, W6/32.HL, greatly decreased SC generation in MLC, but not with rIL-2 or T-cell clones. Thus, the induction of nonspecific SC was dissected into three pathways involving: class I and TU39-defined but not DR, DQ, or DP determinants (in MLC) which was independent of the IL-2R epitope bound by TU69; only TU39-defined determinants (with T-cell clones), which were IL 2R dependent; and, neither class I, class II nor TU39-defined determinants (induction by rIL-2), which was also TU69+ IL-2R independent.