During the past few years, there has been an increasing need for Federal, State and municipal agencies to assess the radioactivity levels in environmental samples such as air, food, vegetation, milk and water. In most cases, it is desired to measure first the gross activity of the sample and then, where indicated, the activity of specific radionuclides. Currently, the measurement of gross activity is complicated by a number of factors such as choice of standard, technique of sample preparation, and type of instrument used for counting. For these reasons, a natural water sample containing a measured amount of pure beta emitting radionuclide was distributed to those members of the Analytical Reference Service who indicated an interest in participating in the study. The report discusses the results of these tests.