Osteosarcomas were induced in mice by three activities of 226Ra (8.8 micronCi, 24.6 micronC and 70.5 micron Ci/kg) and by three activity levels of 224Ra administered in fractionated doses during a period of 75 weeks (total activity per mouse: 0.65 micron Ci, 1.88 micron Ci). In the 226Ra series 74 cases and in the 224Ra series 89 cases of osteosarcomas were evaluated by means of planimetric measurements of roentgenographic images, including the calcified structures. Even though it is not possible to differentiate the series precisely, still some differences between tumors under study could be found. In comparison with 226Ra series, the sarcomas induced by 224Ra are in average rather larger, calcified stuctures appear not to be fully developed and differentiated, he anatomical distribution is different from that characteristic to the 226Ra induced tumors, the mean life span is shorter. It is quite likely that not only the incidence but also some of the features of osteosarcomas are dependent on the time and tissue distribution of the dose delivered.