Radioisotope scanning techniques are now accepted as a valuable adjunct to classical contrast studies in the localization of intracranial tumors. However, advocates of these methods have not approached supratentorial and subtentorial tumors with equal enthusiasm, the latter being regarded as less amenable to localization by scintiscan techniques. Sweet et al1utilizing positron emitters characterized their results with posterior fossatumors as "miserable" (59% missed), and McAfee,2utilizing radioiodinated human serum albumin and scintiscan techniques, obtained "poor" (four of six missed) results with this type of tumor. Early experience here in localization of posterior fossa tumors utilizing scintillation probe multispot survey over the head after administration of I131diiodofluorescein was disappointing in that correct localization was suggested in only three of 11 cases.3More recently we began a program ultilizing further refinements of technique (scanning scintillation detector, effective collimation, and photographic recording) and radiomercury-labeled chlormerodrin. Eighty-four per