The purpose of this single blind clinical study was to determine the effects of two commerical mouthwashes (one containing cetylpyridinium chloride and domiphen bromide and the other containing only cetlypridinium chloride) on existing plaque accumulations. A second purpose was to determine if a residual effect could be shown 2 weeks after cessation of using these mouthwashes. A total of 120 adult subjects, who had been divided into three groups, were initially in the study. For a period of 31 days, the subjects in Group A rinsed with a commercial mouthwash containing cetylpyridinium chloride and domiphen bromide; Group B rinsed with a commercial mouthwash containing only ceytlpyridinium chloride; and Group C served as controls and rinsed with colored flavored water. All subjects continued their normal home oral hygiene practices, except that mothwashes other than as assigned were forbidden. The subjects received three identical examinations to determine their plaque scores. The examinations were conducted the day preceding initiation; the day following cessation; and 15 days after cessation of the rinsing procedures. A total of 105 subjects received the first and second examinations, and 93 subjects received all three examinations. Based upon the data obtained, the daily use of each of the cetylpyridinium chloride mouthwashes tested, appears to be partially effective in reducing existing bacterial plaque accumulations. The cetylpyridinium chloride-domophen bromide product was slightly, but not significantly, more effective than the other commercial mouthwash. Neither commerical product demonstrated a significant residual effect 2 weeks after cessation of use.