Method 1103.1 describes a membrane filter (MF) procedure for the detection and enumeration of Escherichia coli bacteria in ambient water. E. coli is a common inhabitant of the intestinal tract of warm-blooded animals, and its presence in water samples is an indication of fecal pollution and the possible presence of enteric pathogens. The E. coli test is recommended as a measure of ambient recreational fresh water quality. Epidemiological studies have led to the development of criteria which can be used to promulgate recreational water standards based on established relationships between health effects and water quality. The significance of finding E. coli in recreational fresh water samples is the direct relationship between the density of E. coli and the risk of gastrointestinal illness associated with swimming in the water. Method 1103.1 is approved for the detection and enumeration of E. coli in fresh, estuarine, and marine waters. Method 1103.1 and 1603 were submitted to interlaboratory validation in wastewater matrices and study results compared. The estimated percent of total colonies that would have resulted in false positives for disinfected, secondary, and combined disinfected/secondary wastewater were significantly lower for Method 1603 (0.6%, 1.4%, and 1.0%, respectively) compared to Method 1103.1 (6.2%, 11.2%, and 10.3%, respectively). The estimated percent of total colonies that would have resulted in false negatives for disinfected and combined disinfected/secondary wastewater were also significantly lower for Method 1603 (2.5%, and 2.2%, respectively) compared to Method 1103.1 (12.3% and 6.0%, respectively). The estimated percent of total colonies that would have resulted in false negatives were not significantly different for secondary wastewater samples for Methods 1603 and 1103.1 (2.6% and 2.0%, respectively). Based on the high Method 1103.1 false positive and false negative levels when compared to Method 1603, Method 1103.1 is not approved for the analysis of disinfected wastewater. Laboratories wishing to test for E. coli in wastewater using a membrane filtration method are referred to EPA Method 1603. A summary of Method 1103.1 false positive and negative results is provided. Detailed Method 1103.1 and 1603 study results are provided in the validation study reports.