Bone marrow transplantation has been shown to induce donor-specific tolerance in rodent models. This approach could potentially be applied to xenotransplantation across discordant species barriers. To evaluate host factors resisting hematopoietic cell engraftment, we have developed two model systems utilizing the combination of swine into severe combined immunodeficient (SCID) mice. SCID mice lack functional B and T lymphocytes, and can therefore be used to evaluate nonimmune factors resisting marrow engraftment, and for adoptive transfer studies to test the role of immune cells and antibodies. First we transplanted swine bone marrow cells into SCID mice conditioned with whole-body irradiation (4 Gy). For nine weeks following the intravenous administration of 10(8) swine bone marrow cells, up to 3.8% of peripheral blood leukocytes were of swine origin, as determined by flow cytometry (FCM). These cells were all of the myeloid lineage. Swine IgG was also detectable in the serum for up to 14 weeks. The bone marrow of the reconstituted mice contained low percentages of swine myeloid cells, and swine myeloid progenitors could be detected for up to 20 weeks after bone marrow transplantation. In a second model, we grafted thymus and liver tissue from 45-69-day-old swine fetuses under the kidney capsule of 4 Gy-irradiated SCID mice. A suspension containing 10(8) swine fetal liver cells (FLC) was also administered i.p. Long-term repopulation with swine T cells was observed, with up to 1.5% swine T cells detected in the WBC, peritoneum, and spleen for at least 5.5 months postgrafting. These T cells expressed either CD4 or CD8, whereas up to 17.6% of cells in the thymic grafts expressed both CD4 and CD8. The i.p. FLC suspension was required for optimal long-term graft maintenance. Our studies show that (1) low level myeloid and B lymphocyte reconstitution can be achieved by transferring adult swine BMC to irradiated SCID recipients; (2) swine myeloid progenitors were detectable long-term in BMC of these mice, suggesting that stem cell engraftment was achieved; and (3) T cell reconstitution of SCID mice by swine progenitors requires cotransplantation of a swine stromal environment, as is provided by fetal swine thymus/liver grafts. We conclude that nonimmune factors such as those provided by species-specific stromal environments are important for reconstitution of some lineages by discordant hematopoietic stem cells.