During the years 1949–51, the staff of the Medical Division of ORINS spent a major portion of its effort on a study of the therapeutic use of gallium72. This report summarizes the problems which were encountered, the results obtained, the studies completed, and the resulting clinical judgments. Since gallium is still a relatively unknown element and its isotopes are only rarely used in biology, the studies began with problems in physics, proceeded through chemistry, biology, and pharmacology, and ended with an assessment of clinical results. The physical characteristics of the gallium isotopes were restudied. A method of standardization of activity was established for routine use. Methods were developed for tissue assay, and a number of new devices were set up to measure activity. Protection of personnel was a major problem. A device for remotely controlled injection of the active material was designed and built. A urine storage container was constructed, and a system of shielding certain patients was instituted. During the study, a method of preparing gross autoradiograms was developed. Microscopic autoradiograms were not satisfactory because of the short half-life of the isotope and its multiple radiations. Fifteen possibly acceptable compounds of gallium were investigated, of which the citrate proved the best. A number of methods of preparation were used, both to purify materials for pile irradiation and for conversion to the citrate complex for animal and patient use. A method for stable gallium tissue analysis was adapted for use in long-term studies. The toxicity of various compounds was investigated in rats, dogs, swine, and man. These studies included checking the toxicity of other dyes and reagents going into the preparation of the drug as finally used in patients. The distribution and excretion patterns and some elements of metabolic behavior were studied in approximately 400 animals of five different species. Finally, the isotope was used in test doses in 34 patients with various types of cancer. In 21 patients full therapeutic testing was done. The main groups of patients had osteogenic sarcoma with metastases to the lung, or extensive plasmocytoma. The remainder had soft-tissue tumors, metastatic to bone. The interpretation of the therapeutic value of gallium72 based upon these data is as follows: 1. Localization of the Element: When gallium citrate in large doses was injected into the blood stream, it rapidly accumulated in bone and viscera. About 50 per cent was excreted by the kidneys within the first twenty-four hours. Following this period, there was either a redistribution or a differential excretion of the retained material so that at five to ten days after injection there was further accentuation of the differential localization in areas of actively metabolizing bone. There were specific deposits of material in soft-tissue metastases from osteogenic sarcoma.