A study of the bacterial contamination of water in dental units was conducted in five dental practitioners' clinics and one university dental hospital in Tokyo. Various bacteria in numbers of 10(2)-10(5)/ml were detected in the water delivered from all 42 of the dental units examined, though very few bacteria were detected in the tap water upstream of the units. Most of the organisms isolated from the water were Gram-negative bacilli, predominantly Alcaligenes, Achromobacter and Pseudomonas spp. A new decontamination apparatus that could be incorporated into the pipe system of dental units was devised and its performance was evaluated in one unit. It effectively prevented bacterial contamination of the water.