We added spleen cells from adult BALB/c mice treated with total lymphoid irradiation (TLI) to the mixed leukocyte reaction (MLR) using a variety of responder and stimulator cells. The spleen cells nonspecifically suppressed the uptake of [3H]-thymidine and the generation of cytolytic cells regardless of the responder-stimulator combination used. We also examined the effect of the spleen cells on the generation of antigen-nonspecific and antigen-specific suppressor cells in the MLR. The experimental results suggest that the spleen cells from TLI-treated mice inhibit the generation of nonspecific suppressor cells, but do not inhibit the generation of antigen-specific suppressor cells. Thus, alloantigenic stimulation of normal responder cells in vitro in the presence of spleen cells from TLI-treated mice generates large numbers of antigen-specific suppressor cells, but few cytolytic cells or nonspecific suppressor cells. Similar nonspecific inhibition of the MLR was observed with neonatal spleen cells. This in vitro system provides a regulatory model for the induction and maintenance of tolerance in vivo, in which adult mice given TLI or neonatal mice accept allogeneic bone marrow transplants without graft-vs.-host disease.