In 3 experiments with low, medium, and high osteosarcoma expectancy after incorporation of 224Ra or 227Th the osteosarcoma risk of adult (5-6 mth old) female NMRI mice were compared with that of rapid growing (1 mth old) animals. Similar to results in the literature with other bone-seeking radionuclides the age-corrected osteosarcoma incidence was not significantly different in both age groups. In the experiment with high osteosarcoma risk the time of tumor appearance was mainly determined by the time of start of the internal irradiation period. It seems that at lower osteosarcoma expectancy the time of appearance of the tumors is mainly determined by the age of the animals.