We have previously shown that the keys to high-level nontoxic chimerism in syngeneic models are stem cell toxic, nonmyelotoxic host treatment as provided by 100-cGy whole-body irradiation and relatively high levels of marrow stem cells. This approach was unsuccessful in H-2 mismatched B6.SJL to BALB/c marrow transplants, but with tolerization, stable multilineage chimerism was obtained. Ten million B6.SJL spleen cells were infused intravenously into BALB/c hosts on day -10 and (MR-1) anti-CD40 ligand monoclonal antibody (mAb) injected intraperitoneally at varying levels on days -10, -7, -3, 0, and +3 and the BALB/c mice irradiated (100 cGy) and infused with 40 million B6.SJL/H-2 mismatched marrow cells on day 0. Stable multilineage chimerism at levels between 30% to 40% was achieved in the great majority of mice at 1.6 mg anti-CD40 ligand mAb per injection out to 64 weeks after transplantation, without graft-versus-host disease. The transplanted mice were also tolerant of donor B6.SJL, but not third-party CBA/J skin grafts at 8 to 9 and 39 to 43 weeks after marrow transplantation. These data provide a unique model for obtaining stable partial chimerism in H-2 mismatched mice, which can be applied to various clinical diseases of man such as sickle cell anemia, thalassemia, and autoimmune disorders.