Previous studies have shown that alloantigen-activated spleen T cells produce a soluble factor which suppresses mixed lymphocyte reaction proliferative responses, and that the interaction between suppressor and responder cells is controlled by genes of the H-2 complex. However a defect in the expression of suppressor activity was identified in the mouse strain C57BL/6J. Factor prepared from alloactivated B6 spleen cells failed to suppress MLR responses of syngeneic or H-2 compatible responder cells. Unimpaired suppressor factor production by other H-2b strains and failure of suppressor factor production by a B6 congenic strain, B6.C-H-2d isolated the defective gene to the non-H-2 portion of the genome. In addition, the defect appeared to be related specifically to inability to produce an active factor, while the capacity to respond to suppressor molecules was unimpaired. The genetic character of the non-H-2 gene action was identified in F1 hybrid studies. Initially F1 hybrids of the nondefective histoincompatible strains were studied. Suppressor factor from F1 cells suppressed the responses of both parental strains, and parental factors each suppressed the response of F1 cells. Adsorption of F1 factor with Con Aactivated thymocytes of either parental strain removed suppressor activity specific for that strain, leaving activity against the other parental strain intact. The data support cedominant expression and production of distinct, parental H-2 haplotype-specific suppressor molecules by F1 suppressor cells. An F1 hybrid of the defective B6 strain with nondefective BALB/c produced suppressor factor which was also capable of suppressing both parental strains. Production of a suppressive B6-reactive factor by F1 cells was verified by adsorption studies. Thus it appears that non-H-2 genes of the BALB/c parent acted in a genetically dominant fashion to provide the function required for expression of B6 suppressor molecules. We conclude that multiple genes control the expression of alloactivated suppressor cell activity, with at least one gene mapped to the I-C subregion of the murine major histocompatibility complex and one or more genes mapped to the non-H-2 gene complement. © American Society for Clinical Pathology.