A better understanding of the mechanism(s) involved in graft-host-tolerance following allogeneic bone marrow transplantation is needed to develop new strategies to prevent graft-versus-host disease (GVHD). Based on previous studies, mainly in MHC-mismatched donor-recipient pairs, three hypotheses have been proposed: clonal deletion, active suppression and lack of adequate antigen-presenting cells. Our goal was to identify the mechanism(s) by which tolerance is achieved and maintained in radiation chimeras transplanted across minor histocompatibility barriers. Healthy (B6----LP) chimeras were obtained following injection of 10(7) C57BL/6 marrow cells to irradiated (9.5 Gy) LP hosts and used experimentally 100 days after chimerization. The tolerance state of (B6----LP) chimeras could not be abrogated after i.v. transfer of 5 x 10(7) donor-type spleen cells alone or with repeated i.p. injection of host-type antigen-presenting cells. No GVHD was observed when 10(7) marrow cells plus 5 x 10(7) spleen cells from (B6----LP) chimeras were injected to irradiated LP recipients. Chimera spleen cells suppressed GVHD when adoptively transferred to LP recipients of a C57BL/6 graft. These results suggest that in this model the presence of suppressor cells is both necessary and sufficient to maintain graft-host-tolerance.