To evaluate tissue reaction to cobalt 60 irradiation around implant materials used in maxillofacial surgery, 2.0-mm titanium alloy bone screws and 2.0-mm cylindrical solid hydroxyapatite implants were placed in mandibular bone of rabbits and irradiated with a single 15-Gy dose on the fifth postoperative day. Tissue reaction around the implants was examined histologically and histomorphometrically at 7, 14, 28, and 56 days after irradiation. Mature bone was relatively radioresistant, but newly formed bone around both implants was damaged by irradiation in several places. After irradiation, the beginning of bone formation was delayed and the amount of new bone formed was less. Bone-implant contact measured histomorphometrically was less in the irradiated group than in the nonirradiated group. All titanium alloy screws showed bone contact, but two of eight hydroxyapatite implants failed after irradiation, probably the result of loss of primary stability of the cylindrical implants caused by radiation damage to newly formed bone. Keywords: animal study, hydroxyapatite, implants, irradiation, titanium