We investigated the possibility of immunosuppressant-free transplantation of the trachea using high doses of 60Co gamma irradiation of the graft before transplantation. Twenty mongrel dogs were used. Five rings of the trachea were removed from the donors and irradiated with 60Co gamma rays. Five corresponding rings were removed from the thoracic trachea of the recipient dogs, and the irradiated trachea was transplanted. Five animals were placed in each of four dosage groups: group A, no irradiation; group B, 20,000 cGy; group C, 50,000 cGy; and group D, 100,000 cGy. The anastomotic site and graft were covered with a pedicled greater omentum graft. No immunosuppressants were given. In group A, all the animals died within 1 month of tracheal stenosis caused by graft rejection. In groups B and C, one animal in each group survived for a long period, but all the others died of tracheal stenosis caused by graft rejection. In group D (100,000 cGy), the graft became incorporated into the recipient tissue in four of the five animals, and three are still alive (more than 1 year later). These findings indicate that allotransplantation of the trachea without the use of immunosuppressants is possible with pretransplantation irradiation of the graft at the dose of 100,000 cGy.