[en] The periodic measurement of gross radioactivity is being widely used as an indicator of the satisfactory quality of community water systems with respect to radioactivity. Americium-241 apparently is being used by an increasing number of laboratories as an alternative gross alpha activity calibration standard in place of the recommended uranium. Higher alpha counting efficiencies obtained using ²⁴¹Am result in gross alpha activity analysis differences of up to 100% when compared with duplicate sample analyses based on a natural uranium standard. Data presented in this report demonstrate that the use of ²⁴¹Am as an alternative gross alpha calibration standard will, in effect, double the federally specified maximum contaminant level for gross dlpha activity in community water systems. Similar problems are associated with gross beta activity calibration standards but the effects are not as pronounced. Comparative counting efficiency data for ¹³⁷Cs and ⁹⁰Sr/⁹⁰Y as gross beta activity calibration standards are given. Gross beta activity differences up to 25% may be obtained on duplicate analyses depending on the beta calibration standard used. If gross radioactivity measurements are to be useful indicators of water quality, strict adherence to standard procedures and acceptance of single calibration standards used for determining gross radioactivity concentrations in water are needed to achieve industry-wide comparabilty of results