termining radioactivity in streams and water supplies is needed. A variety of methods is in use today by a number of states making measurements of radioactivity present in their streams. It is not very difficult to determine the alpha activity in a water supply (1), but beta radioactivity offers a more complex problem. The types and decay energies of radioisotopes present are generally unknown. Assays from various locations are not comparable and are often meaningless. Brooksbank and Reynolds suggested Tl204 as a standard of comparison for use at the various Atomic Energy Commission plants (2, 3). This isotope has proved to be a reliable tool which gives reproducible results. Reynolds also outlined the use of Tl204 at the Boston, Mass., meeting of the Beta and Gamma Ray Measurements and Standards Subcommittee of the National Research Council, Oct. 2324, 1952. The subcommittee recommended further investigation. The use of this standard will give an efficiency correction for the entire technique employed, without a series of individual corrections for self-absorption, window thickness, and other factors. Wheler, while at the Massachusetts Institute of Technology, used some accurately standardized Tl204, obtained from Reynolds, in making asays of tolerance levels of beta activity in water supplies and found that it gave statistically valid results (4).