Allogeneic reactions are the major limitation to organ transplantation. These are manifested as rejection of the grafted tissue, and also, in the case of bone marrow transplantation (BMT), graftversus-host disease (GVHD)¹. Recent methods of avoiding GVHD,by depleting T cells from donor marrow, have led to an increased incidence of marrow graft rejection^2–5. Current recipient conditioning protocols involving drugs or irradiation cannot safely be increased, so alternatives must be found. Monoclonal antibodies can be used to control immune responses in vivo^6,7, and would be useful in this context if we could define and deplete the cells responsible for marrow rejection. We show here that elimination of residual L3T4⁺ and Lyt-2⁺ cells from mice receiving fully mismatched bone marrow abrogates rejection and promotes tolerance to donor-type skin grafts, even in sub-lethally irradiated recipients.