BALB/c mice given allogeneic (C57BL/Ka) bone marrow cells after toal lymphoid irradiation become stable chimeras approximately 80% donor-type and 20% host-type cells in the spleen. The chimeras doe not develop graft vs. host disease (GVHD). Purified cells of C57BL/Ka origin from the chimeras mediated GVHD in lightly irradiated C3H (third party), but not in BALB/c (host-strain) mice. Thus graft vs. host tolerance in the chimeras could not be explained by complete immunodeficiency of donor-type cells, serum blocking factors, or suppressor cells of host (BALB/c) origin. Clonal deletion or suppression of lymphocytes reactive with host tissues remain possible explanations. The transfer of donor-type chimeric spleen cells to BALB/c recipients given 500-550 rad whole-body irradiation WBI led to stable mixed chimerism in approximately 50% of recipients. The cells were presumably acting as tolerogens because similarly irradiated BALB/c mice given (BALB/c X C57BL/Ka)F1 spleen or bone marrow cells also became stable mixed chimeras.