Isolator-maintained CBH/Ola (Rtlc) rats were lethally irradiated and reconstituted with spleen and bone marrow cells from fully allogeneic WAG or Wistar (Rtlu) donors. Hematopoietically reconstituted rats were treated with cyclosporine (CsA)4 as prophylaxis for graft-versus-host disease (GVHD) for periods ranging from 6 to 26 weeks. Following the termination of CsA treatment GVH reactivity developed in all recipients of allogeneic cells regardless of the duration of immunosuppression. Approximately a third of the reconstituted rats survived the post-CsA period of GVH activity; these rats carried peripheral lymphocytes and spleen cells of donor strain origin and were specifically unresponsive to donor strain skin grafts. Surviving chimeras remained healthy for long periods (up to 18 months) after transplantation, although morbidity increased slightly for rats moved to normal animal house conditions. However, all chimeras had some degree of lymphopenia and showed diminished immunological responses to extraneous antigens and third-party skin grafts. Experiments to elucidate the mechanisms by which specific tolerance was maintained in chimeras indicated that neither the deletion of host-reactive lymphocytes from the graft nor an absence of host bone-marrow-derived "stimulator" cells was responsible. It was shown that the potential GVH reactivity of normal donor strain cells was specifically suppressed in vivo (in the chimera) and that this suppression could be transferred to secondary irradiated recipients by transferring chimeric spleen cells. Attempts to demonstrate a role for suppressor cells in the maintenance of the chimeric state yielded inconclusive results.