[en] The effect of two doses of x-irradiation upon the osteogenesis in isografts or allografts of cancellous bone has been studied. Exposure of rats of 500 rads of x-irradiation caused a transient depression in osteogenesis in the skeleton. This depression was more marked, however, after 600 rads of x-ray. Five hundred rads of x-ray had no effect upon the level of osteogenesis in isografts 2 weeks after grafting, but did depress new bone formation in 3-week-old grafts. The higher dose of x-rays caused a more profound depression in new bone formation at both 2 and 3 weeks. Both 500 rads and 600 rads of x-ray had little effect upon the degree of first phase osteogenesis in H-1 disparate allografts. However, there was some improvement in late phase new bone formation in recipients treated with 600 rads. Both doses of x-rays markedly improved the amount of first phase osteogenesis in H-1 identical allografts. There was also a significant improvement in the new bone formation in grafts in the second phase. The significance of these findings is discussed