The in vitro antibacterial activity of the hydroxyl radical generation system by the photolysis of H₂O₂ in combination with proanthocyanidin, which refers to a group of polyphenolic compounds, was examined. Bactericidal activity of photo-irradiated H₂O₂ at 405 nm against Streptococcus mutans, a major pathogen of dental caries, was augmented in the presence of proanthocyanidin, whose bactericidal effect by itself was very poor, in a concentration-dependent manner. This combination was also proven effective against Porphyromonas gingivalis, a major pathogen of periodontitis. It is speculated that H₂O₂, generated from photo-irradiated proanthocyanidin around the bacterial cells, is photolyzed to the hydroxyl radical, which would in turn affect the membrane structure and function of the bacterial cells, resulting in augmented sensitivity of bacterial cells to the disinfection system utilizing the photolysis of H₂O₂. The present study suggests that the combination of H₂O₂ and proanthocyanidin works synergistically to kill bacteria when photo-irradiated.