Bone biopsy specimens were obtained from 30 cancer patients who had been scanned following injection of strontium chloride Sr 85 or strontium nitrate Sr 85; autopsies were done in 4 additional patients. In 15 cases the85Sr concentration of the specimens was determined. Deposits of85Sr in actively proliferating new bone formed in reaction to tumor invasion. If strontium chloride Sr 85 or strontium nitrate Sr 85 is injected when the reactive process is completed, the scan is normal although the roentgenogram may be dense. Lesions which are radiolucent but scan-normal show no evidence of reactive bone formation. The scan depicts the activity of the reactive process, whereas the roentgenogram shows the net changes which have occurred, both destructive (radiolucencies) and reparative (radiodensities). Tumor cells were found in bone at the site of the abnormal scan in 22 of the 28 patients (79%).