Bone allografts sterilized with ethylene oxide gas (EO) are used in the field of orthopedic surgery, and the reduction of the EO residual concentration is an urgent clinical matter. We therefore investigated the efficacy of aeration and the effects of varied preservation periods and rinsing conditions on the reduction of EO residuals in freeze-dried bone allografts in the present study. Before aeration, the EO residual level was 12.6 ppm, and, after the repeating of aeration at 60 degrees C once, two times, and three times, the level decreased to 10.9 ppm, 3.1 ppm, and 0.47 ppm, respectively. Regarding the duration of preservation at room temperature, the mean EO residual level was 10.5 ppm, 4.9 ppm, and 4.6 ppm, 1, 2, and 3 weeks after EO sterilization, respectively. By rinsing with physiological salt solution, the level was decreased to 6.9 ppm by 5-min rinsing with 100 ml. Rinsing with 500 ml of this solution decreased the levels to 3.9 ppm, 2.8 ppm, and 2.0 ppm when done for 1, 5, and 10 min, respectively. Rinsing with 2000 ml of this solution decreased the levels to 3.6 ppm, 2.6 ppm, and 1.7 ppm when done for 1, 5, and 10 min, respectively. These experimental results with chip bone allografts lead us to recommend repeated preoperative aeration and more than 2 weeks' preservation before use for reducing the residual EO concentration. It was also evident that intraoperative rinsing with 500 ml of physiological saline for 10 min reduced the EO residual level.