Objective: This study was designed to evaluate the efficacy of electrolyzed waters containing different functional-chlorine concentration levels on cariogenic bacteria and biofilm. Methods: Electrolyzed hypochlorite water (PerfectPerioTM water; PPW, Noguchi Dental Medical Research Institute; Noguchi-DMRI, Japan), PPW was diluted; x2 (PPW-2), x4 (PPW-4), x6 (PPW-6) and x10 (PPW-10). Milli-Q water and 0.5% NaOCl were included as controls. Viability of the bacterial cells was assessed by staining with BacLightTM Bacterial Viability Kit followed by fluorescence microscopy and counting colony forming units (CFU/ml) after 10sec PPW treatment. Proteins of PPW treated Streptococcus mutans MT8148 (S. mutans) were inspected by using SDS-PAGE and two-dimensional gel electrophoresis (2DE). Also, biofilms were grown on equal-shaped bovine enamel coupons using four species of oral streptococci (S. mutans, Streptococcus sobrinus, Streptococcus gordonii and Streptococcus mitis) at 37°C for 12 hrs in an oral biofilm reactor. After treatment with PPW the bacteria were separated from three different levels (upper, middle and bottom) of the biofilms by shaking and CFUs/ml counted in each level. Further, to neutralize the functional-chlorine, PPW was diluted with solution of sodium ascorbate (pH7.5). Results: Viability test and CFU data showed that almost all bacteria were killed by the diluted PPW down to the concentration of 100ppm. Protein analysis indicated that S. mutans proteins were damaged on PPW treatment. Interestingly, no colony formation could be detected even in case of samples were plated from the most bottom part of biofilms. Further, it was confirmed that the functional-chlorine concentration in PPW was reduced to 2ppm from 600ppm by sodium ascorbate, but maintained low alkali pH level. Conclusion: PPW maintained as potential bactericide even after penetrating deep into the bottom of the cariogenic biofilms. Also, it was found that sodium ascorbate solution effectively neutralizes PPW. This study was supported by G-COE Program, IRCMSTBD at TMDU and Noguchi-DMRI.