Immunosuppression is believed to play a role in the maintenance of stable bone marrow (BM) chimeras. This study investigates the nature and specificity of the suppression that lymphocytes from allogeneic BM chimeras exert upon the alloreactivity of donor and recipient lymphocytes. Lethally irradiated CBA/J (H-2k) mice were infused with 10(7) unseparated (WBM) or T cell-depleted BM (TDBM) cells of B10.BR mice (H-2k, disparate at minor histocompatibility antigens). Mixtures consisting of spleen cells (SC) from BM chimeras and SC from either normal donor, recipient, or third party (C3H, H-2k) mice, were sensitized with irradiated BALB/c (H-2d) leukocytes, then assayed for proliferative and anti-H-2d cytotoxic activity and compared with those of appropriate control cultures. The alloreactivity of all three types of normal SC was non-specifically suppressed by SC from both WBM and TDBM chimeras taken 2 weeks post-BM transplantation (BMT). In contrast, at 4 weeks post-BMT, SC from both chimeras suppressed the alloreactivity of recipient-type cells whereas only SC from WBM, but not from TDBM chimeras, suppressed normal donor-type response, and neither could suppress the response of normal third party cells. The suppression of donor-type alloreactivity diminished with time, while that exerted on recipient-type lasted for at least 10 weeks post-BMT. The suppression of donor alloreactivity was mediated by radioresistant Thy1.2+, Lyt1+2+ cells while that exerted upon recipient's alloreactivity was mediated by radiosensitive Thy1.2+, Lyt1+2- cells. Both types of suppressor cells were of donor origin. The potential biological role of the suppressive activity in the engraftment of allogeneic BM is discussed.