A 5 day topical kanamycin treatment of the dental gingival surfaces in institutionalized mentally retarded subjects significantly reduced the plaque mass on the tooth surfaces compared to a placebo treatment. This reduction occurred in the absence of any mechanical hygiene procedures and lasted for 4 or more weeks, which was longer than the kanamycin could have been expected to be present in the mouth. This suggested that the kanamycin had caused an alteration of the plaque flora which should be discernible by quantitative culturing procedures. Plaque was removed and cultured, using an anaerobic serial dilution procedure, before and after kanamycin or placebo treatments. The reduction in plaque weight was associated with reduced levels of streptococci in the plaque. This could be demonstrated by intergroup comparison of the placebo ‐ and kanamycin treated subjects, and by intragroup comparisons of the kanamycin treated subjects prior to treatment and at 4 and 8 weeks after treatment. The streptococci species accounted for 30% of the plaque colony forming units (CFU) prior to kanamycin treatment. At 4 weeks after treatment, the streptococci had decreased to 3% of the CFU and at 8 weeks after treatment, the streptococci had increased slightly to account for 14% of the CFU. The streptococci appeared to beStreptococcus sanguisandStreptococcus mitis. The observations indicate that the topical kanamycin was effective in reducing supragingival plaque because of its ability to decrease the proportions of streptococci in the plaqu