Thymic tissue transplantation has been performed previously in adult mice to induce donor-specific tolerance across allogeneic and xenogeneic barriers. We have now attempted to extend this technique to a large animal preclinical model and describe here our initial studies of allogeneic thymic transplantation in miniature swine. Two miniature swine were thymectomized before thymic tissue transplantation, and two remained euthymic. Donor thymic tissue was harvested from SLA class I-mismatched juvenile pigs and placed into recipient sternocephalicus muscle, kidney capsule, and omentum. A 12-day course of cyclosporin A was started on the day of transplantation. Allogeneic thymic engraftment could only be achieved in euthymic and not in thymectomized miniature swine using this treatment regimen. Both nonthymectomized animals showed good graft development, with evidence of thymopoiesis, as indicated by positive CD1 and host-type SLA class I immunoperoxidase staining of immature graft-infiltrating cells. Both animals also demonstrated donor-specific T cell hyporesponsiveness, as measured by MLR and cell-mediated lympholysis. The thymic grafts continued to develop despite the appearance of high levels of anti-donor specific cytotoxic IgG Abs. Thus, thymic tissue transplanted across an SLA class I barrier can engraft and support host thymopoiesis in euthymic miniature swine. The presence of the host thymus was required for engraftment. These data support the potential of thymic transplantation as part of a regimen to induce donor-specific tolerance to xenogeneic organ grafts.