The radial distribution of CFU-S in the femur of a normal 8 weeks old mouse is shown and corresponds closely to Lord and Hendry. The highest concentration (about 41/10,5, cells) was near to the bone surface and the lowest (about 22/10,5, cells)was close to the central venous sinus which runs roughly along the longitudinal axis of the bone shaft. Continuous exposure to γ-rays for 6 d from 13d of gestation to birth had a smaller effect on the total femoral cellularity at 8 weeks of age, decreasing it by 12%~20%. The 15 cGy/d regimen caused a significant(20%) CFU-S deficit (,p,<,0.02) in the marginal zones. The 60 cGy/d schedule increased the deficit to a level (33%) and the change in the femoral CFU-S distribution after γ-ray irradiation is also shown. The 60 cGy/d schedule also reduced the fibroblastoid, CFU-F population, to 67% of control (,P,<,0.001). The 15 cGy/d reduced to 88% of control.