Information has been assembled on the frequency of occurrence of osteosarcomas in the long bones of groups of beagles irradiated by alpha particles from bone-seeking radionuclides. These data are compared with measured trabecular surface areas in the long bones of a beagle, determined at the Bone Dosimetry Research Laboratory, Cookridge Hospital, Leeds. It is shown that, as in the human case, the percentage tumour occurrence in a long bone (or in separate thirds of a long bone) was linearly related to the corresponding trabecular surface area with a high value of the correlation coefficient. For single injections of 226Ra the low percentage tumour occurrence in the mid-thirds of the beagle bones was close to that found in man. For the surface-seeking radionuclides (239Pu, 228Th) and for 228Ra much higher percentages of tumours were found in the mid-thirds of the dog bones than for 226Ra injection. When, however, 226Ra was given in eight fractions with 14 days between each injection, a very high tumour incidence was found in the mid-thirds of the beagle long bones. These results are discussed in the light of dosimetric and other relevant factors.