Deep-frozen morselized human bone grafts showed osteoconductivity and osteoinductivity when implanted into tibial window defects of nude rats. The osteoconductivity was assessed by measuring the total area of newly formed bone bridged by the implanted bone grafts in the entire defect area. The osteoinductivity was evidenced by the presence of active osteoblast-like cells and new bone formation around the implanted bone grafts, which were surrounded by soft tissues distant from the host cortex. Gamma irradiation at the doses of 15 or 25 kGray reduced the osteoconductivity (ANOVA and LSD tests, p<0.05) at 3 weeks post operation. The 25 kGray group had a significantly lower level of new bone formation compared with the 0 and 15 kGray groups. The evidences of osteoinductivity were only noted in the 0 and 15 kGray groups. Our data indicate that 25 kGray gamma irradiation reduces the osteoconductive and osteoinductive properties of the morselized human bone graft.