Preparative regimens for marrow allografts in thalassemia have two objectives. One is eradication of diseased marrow and the other suppression of host-versus-graft (HVG) reactions so that the allograft survives. A common regimen to accomplish these goals has combined high-dose busulfan with cyclophosphamide. Postgrafting immunosuppression with cyclosporine/methotrexate has been used for GVHD prevention. Some patients may die from regimen-related toxicity. Overall event-free survival is 75%. Occasional patients have become mixed donor/host hematopoietic chimeras and, yet, disease symptoms have abated. This has raised the possibility of developing safer and less toxic transplant programs that result in stable mixed hematopoietic chimerism. We have devised such a program in dogs consisting of a nonlethal dose of total body irradiation (200 cGy) before and a novel combination of mycophenolate mofetil and cyclosporine after transplant. Mixed donor/host chimerism (> or = 50% donor cells in all lineages) has persisted for > 80 weeks, even though immunosuppression was discontinued after five weeks.