Methods based on the measurement of beta-glucuronidase have been shown to be specific and inexpensive for the identification of Escherichia coli from bacterial colonies within 1 h. Recently, commercial systems incorporating beta-glucuronidase substrates were introduced. Rapid Identification Method E. coli (Austin Biological Laboratories, Curtin Matheson Scientific, Inc., Houston, Tex.) and Rapid Detect E. coli (Organon Teknika, Morris Plains, N.J.) are single-tube test combinations to simultaneously measure beta-glucuronidase (fluorescence at 366 nm), o-nitrophenyl-beta-D-galactopyranoside (yellow), and indole (red). To determine the accuracy and utility of these two systems, we used them to test 169 E. coli and 150 non-E. coli and compared them with conventional substrate tests. The Rapid Detect test was more efficient than the Rapid Identification Method in demonstrating beta-glucuronidase activity, but the commercial systems were equal to each other and to the conventional tests for o-nitrophenyl-beta-D-galactopyranoside and indole. There were no false reactions by either system.