Allogeneic, H-2-incompatible irradiation chimeras (H-2d leads to H-2b) constructed with normal, unmanipulated bone marrow and with marrow-derived factors live long and do not manifest a GvH disease. Their response to primary immunization is deficient but their alloreactivity is normal. This chimeric allotolerance cannot be passively transferred from chimeric donors to normal irradiated recipients. Passive transfer of both donor- or recipient-type immunocompetent T-cells into the chimeric mice does not lead to syngeneic reconstitution, rejection of the engrafted marrow or GvH disease and the mice maintain permanently their chimerism. This new model demonstrates that chimerism is not eradicable in long-lived chimeras reconstituted with unmanipulated bone marrow, and that the bone marrow itself plays a dominant role in maintenance of chimerism.