In this study we evaluated (1) the efficacy of a protocol that combines hydrogen peroxide (shock treatment) and ICX® tablets (continuous treatment) for the control of microbial contamination in dental unit water lines, and (2) the in vitro antimicrobial activity of ICX® tablets on collection and wild strains isolated from dental chair output waters. To assess the treatment effectiveness, the microbial load in the output water samples of three dental chairs were investigated: one control chair received only shock treatment. In vitro bactericidal activity was tested against Staphylococcus aureus and Pseudomonas aeruginosa. Data obtained from samples collected from chairs treated with ICX® and shock treatment and data from the control chair did not differ significantly on the basis of microbial load. In the in vitro study, the product was unable to kill Gram-negative bacteria. These results show that the continuous introduction of ICX® was not effective in maintaining low counts of the heterotrophic bacteria in the output water of dental devices, and shock treatment may be needed more frequently than monthly.