The efficacy of a dental unit equipped with a system that disinfects and sterilizes the water tubing by flushing with glutaraldehyde was evaluated by inserting Bacillus megaterium spores and Pseudomonas and Moraxella species into the water tubing. Up to 10 to the 8th power Pseudomonas a nd Moraxella organisms were killed during the disinfection cycle, but Bacillus megaterium spores were not. Up to 10 to the 5th power spores were eradicated by the sterilization cycle, although the system did not consistently kill 10 to the 8th power spores. The water tubing of the new unti was not naturally colonized by water bacteria during an 8-month period prior to the study. Evidence suggested that this was due to antimicrobial activity associated with the plastic tubing: therefore, microbial contamination of new dental units, irrespective of their design, would not be expected, until the inhibitory factor in the plastic tubing has leached out.