Methods for the microbiological analysis of drinking water must be able to detect Escherichia coli that may be injured by treatment. The Colilert method, which simulaneously detects total coliforms and E. coli in water samples by the observation of direct colour changes produced by defined substrates in the media, was found to be equivalent to the reference EC MUG method in its ability to recover low numbers (< 4/100 ml) of oxidant-stressed E. coli.