Interest in rapid bacterial detection methods for sanitary indicator bacteria in water prompted a study of the use of (14)C-mannitol (UL) to detect fecal coliforms (FC). A simple method was developed using m-FC broth, membrane filtration, and two-temperature incubation (35 C for 2 h, followed by 44.5 C for 2.5 h). Results indicated that FC numbers ranging from 1 x 10(1) to 2.1 x 10(5) cells could be detected in 4.5 h. Within sample reproducibility at all cell concentrations was good, but sample to sample reproducibility was variable. Comparisons between m-FC broth and m-FC broth modified by substituting 4-mannitol for lactose indicated that the standard m-FC broth was the better test medium. Results from experiments employing dimethyl sulfoxide (DMSO) to increase permeability of FC to (14)C-mannitol indicated no increase in (14)CO2 production due to DMSO. Detection of FC using this method may be useful for rapid analyses of potable water suspected of contamination due to distribution line breaks or cross-connections, or for quick surveys of potable source water quality.