The effects of dose fractionation on the survival rate, the numbers of bone marrow cells (BMC) and hemopoietic stem cells (CFU-S) per femur, and the self-renewal capacity of the CFU-S in ICR/JCL and LACA mice were investigated. It was found that dose fractionation affected the mouse survival rate significantly. At 2 months after 18 Gy irradiation, the survival rates of mice received 6 doses of 3.0 Gy and 4 doses of 4.5 Gy were about 40% and 15% respectively, but all mice died after 3 doses of 6.0 Gy. When the total dose of 12 Gy was split into 2-4 fractions, most mice survived more than 5 months. Although the number of BMC per femur recovered to normal or near normal level after 2 months of irradiation, the number of CFU-S per femur was only about 40% and the self-renewal capacity of the CFU-S was only about 25% of the age-paired controls. This kind of hemopoietic residual damage remained unchanged up to 11 months after irradiation and no significant differences were found among the various dose fractionation groups.