Small bowel transplantation is associated with a significant risk of graft versus host disease owing to the large amount of organized lymphoid tissue within the graft. This study assessed whether graft lymphoid cells could persist in the long term following fully allogeneic small bowel transplantation when graft rejection was prevented by cyclosporin immunosuppression. Transplantation was carried out between PVG and DA strains of rat. Cyclosporin (15 mg/kg) was given daily from transplantation, and groups of animals were studied at 28 and 56 days after grafting. The proportions of donor- and recipient-derived cells in the graft and in the host gut and lymphoid tissues were assessed using immunohistochemical tissue staining and monoclonal antibodies specific for cells expressing class I antigens from the two strains of rat. Results demonstrated a persisting population of graft-derived T cells which were capable of migration to the host. Therefore, there may be a long-term risk of graft versus host disease after small bowel transplantation under cyclosporin immunosuppression.