The supplementary effect of chlorhexidine varnish and resin on the reappearance of mutans streptococci and Actinomyces spp. on root surfaces after chlorhexidine gel treatment was studied. In 8 subjects with many restored tooth and exposed root surfaces, highly colonized with mutans streptococci, chlorhexidine varnish and resin were applied to the teeth on one side of the mouth and resin alone to the other side. This treatment significantly prolonged the time period of reduced salivary levels of mutans streptococci when added to a preceding period of chlorhexidine gel treatment. In 3 of the 8 subjects, mutans streptococci were undetected for more than 4 weeks after treatment. There was a significant difference between the chlorhexidine varnish/resin side and the resin side for 5 weeks with regard to the mean numbers of mutans streptococci in root surface plaque, whereas for the Actinomyces spp. no significant differences were found. In 2 subjects harboring both Streptococcus mutans and S. sobrinus, the population of S. sobrinus reappeared more readily in saliva and plaque than S. mutans. The chlorhexidine treatment was less effective in suppressing the population of Actinomyces viscosus/naeslundii than the mutans streptococci.