Thymus derived lymphocytes (T cells) from mouse spleen, activated in vitro or in vivo with concanavalin A (Con A), suppress proliferative responses of syngenic lymphocytes in mixed lymphocytes reactions (MLR). Replication in vitro was not required for expression of suppressor activity by Con A activated cells and was blocked in MLR by treating suppressor cells with mitomycin C or irradiation. Kinetics of MLR responses and viability of cultures were not altered by addition of activated suppressor cells. The data are consistent with a direct inhibitory effect of suppressor T cells on antigen induced DNA replication. These observations extend a model previously described for the regulation of antibody synthesis by Con A activated T cells to control cell mediated immune responses. This model should be particularly useful in further definition of regulatory T cell subpopulations, and investigation of interactions and relationships between such populations.