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A medium containing a chromogenic substrate was developed for the enumeration of Escherichia coli on the basis of l-glucuronidase activity. In this medium there was an inverse linear relationship between the log initial E. coli concentration and the time taken for the color to reach a threshold optical density of 0.05. This relationship applied even when the E. coil population contained 5% 0-glucuronidase-negative cells. Incubation at 44°C reduced the time taken for color development and aUlowed the procedure to be used in the presence of a competitive microflora that outnumbered the E. coil population by a factor of 104. Sodium lauryl sulfate as an additional selective agent gave no significant improvement. In the analysis of environmental water samples, the technique gave a good correlation with a standard cultural method. The procedure shows promise as a simple method for testing the compliance of environmental samples with microbiological criteria for E. coli.

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