Bone marrow transplantation in human X-linked severe combined immunodeficiency (XSCID) without pretransplant conditioning results in engraftment of donor T cells and reconstitution of T-cell function but engraftment of few, if any, donor B cells and poor reconstitution of humoral immune function. Since bone marrow transplantation remains the most effective treatment of XSCID patients, better strategies are necessary to achieve optimum long-term results. Canine XSCID, like human XSCID, is due to mutations in the common gamma chain (gamma c) gene and has clinical and immunologic features identical to those of human XSCID, making it a true homolog of the human disease. We have successfully performed bone marrow transplantation in three XSCID dogs without pretransplant conditioning, using untreated bone marrow cells from mixed lymphocyte culture-nonreactive normal littermates. Unlike the experience in human XSCID patients, all three dogs engrafted both donor B and T cells and attained full reconstitution of immunologic function. Normal percentages of T cells and T-cell mitogenic responses were attained by 3 months posttransplant. CD3+ T cells after transplantation expressed the CD45RA isoform indicating that the cells were recent thymic emigrants derived from immature progenitors. Serum IgG levels were within normal range by 5 months posttransplant. Immunization with the T-dependent antigen, bacteriophage phiX174, demonstrated normal antibody titers, immunologic memory, and class-switching. Polymerase chain reaction (PCR) analysis of the gamma c locus showed that 100% of circulating T cells and 30% to 50% of circulating B cells were donor-derived. None of the dogs developed clinically evident graft-versus-host disease (GVHD). Thus, canine XSCID provides a model to determine the optimal conditions for bone marrow transplantation in human patients, and to develop and test strategies for somatic gene therapy.