[en] Conditioning protocols were tested for their efficacy in increasng the incidence of engraftment of histoincompatible dog bone marrow cells. Cyclophosphamide and total body irradiation (TBI), Corynebacterium parvum and TBI, a 3- or 5-day delayed transfusion of bone marrow cells after TBI, or an increase in the number of donor bone marrow cells or lymphocytes appeared to be ineffective. These protocols were previously reported to promote recovery of splenic hemopoiesis in mice in short-term assays. The noted discrepancy between studies with mice and dogs invalidated allogeneic resistance as measured in the mouse spleen assay as a model for bone marrow allograft rejection. Intravenous treatment with silica particles or L-asparaginase did improve the engraftment rate after 7.5 Gy TBI. Low efficiency and significant extra toxicity restrict the applicability of these procedures. The most promising conditioning schedule found appeared to be two fractions of 6.0 Gy TBI separated by a 72-h interval. Prolonged survival was noted after transplantation of bone marrow cells from a one-DLA haplotype-mismatched donor. Possibilities for further improvement of this protocol are discussed