The influence of a mouthrinse containing 0.03% triclosan (2,4,4'-trichloro-2'-hydroxydiphenyl ether) and 0.25% copolymer (polyvinylmethylether maleic acid copolymer), compared to a matching placebo rinse, on in vitro plaque formation was assessed in two dynamic plaque model systems. In the controlled saliva flow cell system, the triclosan-containing rinses significantly reduced total plaque compared to the placebo rinse, with no significant differences when copolymer was removed from the active rinse formula. In the continuous culture system (chemostat flow cell model), the mean plaque was significantly reduced when compared with placebo. The results of these studies indicate that a triclosan mouthrinse, in the presence or absence of copolymer, is effective in reducing plaque in vitro.