application of nuclear energy and its byproducts have focused increasing attention upon the public health problems connected with the pollution of water by radioactive material. Liquid wastes from the operation of nuclear reactors, wastes from the uses of radioisotopes, and fallout from the detonation of nuclear weapons already have added quantities of radioactivity measurable above the natural or background radioactivity in water. The USPHS is collaborating with the AEC and other agencies in delineating the problems and assisting in their solution. One of the responsibilities of the Robert A. Taft Sanitary Engineering Center is to investigate and develop methods for the measurement of radioactivity in water, sewage, and industrial wastes. The purpose of this article is to describe one reliable method.