In order to develop an enterococci assay method as a pollution indicator of water or waste water, the most probable number (MPN) method using BTB azide dextrose and the membrane filter (MF) method using m-enterococcus agar were compared as to usefulness and simplicity. The appropriate concentration of NaN(3) in the media to permit selective growth of enterococci was determined to be 0.025% NaN(3) in the case of the MPN method, and 0.04% in the case of the MF method. Three hundred specimens from river water were tested by the MPN and MF methods. The correlation coefficient between the results was high enough to draw the conclusion that both methods were useful. The MPN method can be recommended as a simple procedure, and the MF method as an accurate assay, especially when specimens contain small numbers of bacteria.