The skeleton is irradiated in persons containing Thorotrast as a result of the deposition therein of the ThO2 colloidal particles themselves and of bone-seeking thorium-series radionuclides released into the blood stream from Thorotrast deposits elsewhere in the body. It has been believed that dose-effect relationships in the skeletons of the Thorotrast population might usefully supplement similar data from populations irradiated by internal deposits of 226Ra, 228Ra or 224Ra. The concentrations of these radionuclides in the skeleton in vivo may be estimated directly from measurements on bone samples collected at autopsy; for the radium isotopes they may also be derived from measurements on the release of these isotopes from the primary Thorotrast deposits in other tissues combined with calculations of skeletal deposition based on the current ICRP model of alkaline earth metabolism in adult humans. Measurements in the laboratory of the IAEA indicate that average bone dose rates in typical subjects systemically injected with 50 ml Thorotrast would be about 1 rad/yr for each of 3 sources: Thorotrast itself, 228Ra continually deposited from blood, 224Ra continually deposited from blood. The 224Ra, by virtue of its deposition on bone surfaces, is toxicologically the most interesting source; dose levels are high enough to sustain the earlier belief that the Thorotrast populations may contribute useful information on the toxicity of skeletal irradiation.