Four cases of osteosarcoma were studied utilizing 85Sr macroautoradiography correlated with the findings of roentgenography, bone scans, and gross and microscopic pathology. The technique of obtaining macroautoradiographs is described. The relationship of the localization of 85Sr to the histological pattern of bone formation is discussed. It is concluded that the highest rates of isotope uptake occur in areas of greatest tumor cellularity rather than in areas of greatest density of tumor bone. Sites of calcification and ossification in tumor cartilage also showed marked avidity for radioactive strontium.