Summary. Graft rejection and graft-versus-host disease are major problems in mismatched marrow transplants along with toxicity from standard myeloablative host treatments. We have established a tolerization model, using 1 Gy irradiation, which reduces stem cell capacity to < 10% of control while causing minimal myelosuppression, donor antigen pre-exposure (spleen cells), CD40-ligand antibody blockade and high levels of marrow (40 × 106 cells), which allows for stable long-term multilineage engraftment in H2-mismatched murine marrow transplants. We now show that the establishment of ‘microchimaerism’ (0·5–3·8%) sets the stage for macrochimaerism, with subsequent marrow infusions in H2-mismatched mice with CD40-ligand blockade only. Neither irradiation nor spleen cell exposure were necessary. When 40 x 106 bone marrow cells were infused on weeks 0, 12, 14 and 16, blood engraftment was about seven times the single 40 × 106 control. When marrow cells were given on weeks 0, 3, 4, 5 and 6, engraftment at 24 weeks post transplant was 17·9 ± 1·2%, compared with 2·7 ± 0·8% for the single 40 × 106 control (P = 0·009). We have shown stable, long-term multilineage chimaerism and established that the schedule of marrow administration, not the total cell dose, is critical for tolerization. This approach indicates that microchimaerism can tolerize for subsequent marrow infusions and produce macrochimaerism. This strategy could be applied in clinical human transplants.