Aplastic anemia was produced with 32P in rabbits with and without splenectomy. Transient hemopoietic engraftment without graft versus host (GvH) was seen after conditioning with antilymphocyte serum (ALS) and infusion of allogeneic marrow from a strain that is known to cause fatal secondary disease if total body irradiation is used for conditioning. More takes could be achieved in the splenectomized group but in both groups the takes were only transient. The absence of GvH was shown to be probably due to a split chimerism of the immunologic system. The graft rejections are felt to be due to the persistence of host T cells. Possibly the duration of engraftment can be prolonged in the future by continued immunosuppressive therapy after transplantation with ALS or cyclophosphamide. The fact that the spleen is not necessary for hemopoietic recovery after i.v. marrow infusion may have important clinical implications because many patients with aplastic anemia are splenectomized in the course of their disease