The usefulness of Electrolized Oxidizing Water as a chemical plaque control agent was investigated. The subjects were 30 female volunteers ranging from 15 to 25 years of age, in whom the Plaque Index and Gingival Index were controlled to zero level before initiation of the study. The subjects were divided into three groups, ordered to rinse their mouth with Electrolized Oxidizing Water, distilled water or 0.05% CPC during the study interval, and not allowed to use a toothbrush or other cleaning devices during this period. The following clinical parameters were measured 0, 3, 7 and 10 days after initiation of the study ; 1) Plaque Index, 2) Gingival Index, 3) Gingival crevicular fluid level, 4) Bacterial flora, and 5) Halitosis. As a result, suppression of plaque increase, prevention of gingivitis and prevention of halitosis were noted after mouth rinsing with Electrolized Oxidizing Water and 0.05% CPC. These findings indicate that Electrolized Oxidizing Water ingredients are useful as a chemical plaque control agent.