Tracheal transplants were performed on inbred strains of rats. Monoclonal antibodies were used in combination with the biotin-avidin technique for demonstrating donor-specific RT1 Ac transplantation antigen. Transplantation antigens were predominantly contained in the tracheal mucosal cells. No donor-specific antigens could be found in the mucosa of transplants which survived orthotopic tracheal transplantation for longer than 210 days. This suggests that the mucosa of the transplant is rejected and replaced by a mucosa from the recipient.