1. Homogenous-bone grafts in dogs were studied by implantation of radioactive bone from donor dogs previously injected with Ca⁴⁵ and by implantation of non-radioactive bone into hosts which were injected with Ca⁴⁵ prior to, during, or after the grafting procedure. Determinations of serum and bone specific activities were made and compared.

2. A procedure for quantitative autoradiography was developed and applied to the analysis of cross sections of grafted and non-grafted areas.

3. The activity in cross sections of the shafts of the long bones consisted principally of a general diffuse component, the specific activity of which was relatively constant over the 100-day period of study and was proportional to the administered dose. Upon this diffuse component were superimposed a few intense hot spots and often a ring of increased activity in or near the endosteum or periosteum. The hot spots corresponded to individual Haversian systems, and their specific activities ranged up to thirty-five times that of the diffuse component as determined with an autoradiographic resolution of 120 micra.

4. The distribution of Ca⁴⁵ from radioactive grafts were systemic and no preferential transfer of Ca⁴⁵ to callus or nearby bone was detected.

5. In dogs which received non-radioactive grafts and injections of Ca⁴⁵, the specific activity of the callus was the highest observed anywhere in bone, up to 100 times that of the diffuse component of the host's cortical bone and was strongly dependent on the relation between the time of injection and the time of grafting.

6. The time of calcification of callus occurred predominantly at about the third week after grafting.

7. Relocation of localized deposits of activity without dilution in the whole volume of circulating blood (hot-spot migration) to or from graft areas was not detected in this experiment.

8. The data of this experiment provided two points which are considered pertinent and applicable to the problem of radiation hazard evaluation with respect to radioactive materials fixed in bone. These are the proportionality between the administered dose and the specific activity of the diffuse component and the large ratio between the specific activity of hot spots and that of the diffuse component.