The efficacy of a commercially available defined-substrate technology (Colilert) was compared with that of a method approved by the US Environmental Protection Agency for enumerating chlorine-treated Escherichia coli from raw sewage samples. A total of 19 samples were analyzed using a protocol provided by AWWA's Filtration and Microbial Technical Advisory Workgroup. For 18 of the 19 samples analyzed, a greater number of positive tubes was found using the commercially available test. Statistical analysis of the data indicate that this test yielded a significantly higher number of positive tubes for chlorine-treated coliform bacteria and E. coli when the disinfectant was chloramine. When free chlorine was available during the chlorine treatment, the recovery of E. coli was equal for the two methods. Includes 23 references, tables.