We investigated osteogenesis and lymphocyte subsets in xenogeneic bone transplantation, using the immunosuppressant FK506 (FK). Iliac bones of rabbits were transplanted as fresh and frozen xenogeneic bone grafts into an intramuscular pouch of rats. FK was injected intramuscularly in half of the rats in a dose of 3 mg/kg/day for 14 days after transplantation. At 2, 4, and 8 weeks, transplanted grafts and the lymphocyte subsets of these rats were examined. In the group not given FK, the grafted bone became necrotic and infiltrated with small round cells around the trabeculae. In the FK group, at 2 and 4 weeks, new bone was formed in the fresh xenografts without infiltration of lymphocytes. At 8 weeks, the new bone became necrotic and lymphocytes were present. The percentage of T cells (CD 5), B cells and the ratio of CD 4 cells/CD 8 cells were smaller in the FK group. Using an immunosuppressant we concluded that xenogeneic bone has an osteogeneic potency.