Objective: To evaluate the antimicrobial potential of BreathRx Anti-Bacterial Mouthrinse using the Log Reduction Assay as specified by the FDA (1994). Methods: Fresh cultures of S mutans (ATCC 25175), A. viscosus (ATCC 19246) and C. albicans (ATCC 18804) were prepared for each experiment. Lecithin and polysorbate 20 were used as neutralizers because BreathRx (Discus Dental Inc., Culver City, CA) is a quaternary ammonium-based mouthrinse. BreathRx and microbial culture were added to media with or without the neutralizers to determine the effectiveness of the neutralizers. The effect of neutralizers on microbial viability was examined by culturing the microbes in media with or without the neutralizers. To determine the bactericidal potential, the mixture of BreathRx and microbial culture was incubated at 37oC for 10 minutes; sample aliquots were diluted in the media with neutralizers; and the mixture was then plated. For evaluating the Bacteriostatic potential, samples were processed using the same procedures for the bactericidal assay, except that the mixture was incubated at 37oC for 48 hours before plating. Counts of surviving cells were determined using the plate-count method. Results: The data obtained from the assays for evaluating effects of neutralizers on the active ingredient in BreathRx and on the viability of test microbes showed that the neutralizers used were effective and they did not have significant adverse effect on the microbial viability; therefore the assays were validated. The log reductions of BreathRx in the bactericidal assay were 3.58, >5.50 and >4.61 for S. mutans, A. viscosus and C. albicans, respectively, while the log reductions of BreathRx in the bacteriostatic assay were 5.22 for S. mutans and >5.50 and >4.61 for A. viscosus and C. albicans, respectively. Conclusion: BreathRx Anti-Bacterial Mouthrinse has significant antimicrobial potential and meets FDA requirements as determined using the log reduction method. Supported by Discus Dental, Inc.