A low nutrient medium, dilute peptone, and reduced incubation temperatures (25 or 30 degrees C) were used to recover bacteria from dental unit water supply. Significantly greater numbers of bacterial colony-forming units were recovered on the dilute peptone medium than on the enriched media, blood agar or trypticase soy agar. Lower incubation temperatures yielded greater numbers of colony-forming units on all media. The bacterial population in dental unit water supply following stagnation in the supply lines and flushing of the lines was studied using dilute peptone incubated at 25 degrees C. No significant differences in the numbers of colony-forming units were found in stagnant water versus fresh water. Flushing the water lines for 10 min did not significantly reduce the numbers of colony-forming units.