The regulation of B-cell and T-cell immune responses has been extensively examined and in the experimental animal appears to involve regulatory or "suppressor" T cells (1-4). The limitations of in vitro experimentation have made comparable study of nonpathological human suppression quite difficult (5). We report here an in vitro method that generates and quantitates suppressor activity in man after antigen-specific activation in mixed leukocyte culture (MLC). The one-way MLC induces both a proliferative response (6) and the generation of cytotoxic T lymphocytes (CTLs) (7). Both of these responses are mediated by antigen-specific T-cell subpopulations (8,9) and have been correlated with recognitive and destructive phases of allograft rejection. Recent reports have examined the antigen reactivity of mouse (10,11), rat (12), or human (13,14) lymphocytes obtained after proliferation in MLC. In all cases, after the primary MLC proliferative peak, the recovered lymphocytes rapidly differentiate upon re-exposure to the initial stimulating population, but do so only weakly when exposed to a presumably noncross-reactive third-party stimulating population. Velocity sedimentation separation studies have shown that the blast cells produced in a primary MLC revert to small lymphocytes that rapidly differentiate into proliferating and/or cytotoxic T lymphocytes upon restimulation with the initial antigen (15). These findings demonstrate that positive selection for the responding population in primary MLC does exist and may account for at least part of the specificity of the secondary response. However, this positive selection does not preclude possible involvement of a suppressor mechanism. In fact we have detected suppressor activity in primary MLC sensitization cultures at a time when the proliferation responsible for positive selection does not preclude possible involvement of a suppressor mechanism. In fact we have detected suppressor activity in primary MLC sensitization cultures at a time when the proliferation responsible for positive selection in not yet significant, suggesting that suppression may be overriding importance in the specificity of MLC-activated secondary responses.