The present study is concerned with the use of a radioactive mercury compound in the diagnosis and localization of brain tumors. The results in over 900 patients, 80 with brain tumors, were analyzed. In 61 cases accurate localization was obtained. In eight cases the results were equivocal and in 11, negative. The technique is more valuable with supratentorial tumors. Certain tumors such as glioblastomas and meningiomas give the highest incidence of satisfactory results. The method is useful with many other tumors but a few, such as epidermoid cysts, have given negative results. Lesions other than tumors, granulomas, angiomas, and cerebral infarcts may also give positive results and must be differentiated from tumors by other means. The method is relatively simple, painless, and associated with little or no risk.