Twenty male and female adult subjects were entered into a 7-week, double-blind clinical study to determine the effect on plaque formation of a mouthrinse containing 0.03%[corrected] triclosan and 0.25% of a copolymer of methoxyethylene and maleic acid, as compared to 1) a water placebo mouthrinse, 2) a matching alcohol placebo mouthrinse, and 3) Plax antiplaque pre-brushing dental rinse. The subjects were stratified according to their initial plaque scores and assigned in a type of randomized block design (repeated Latin square), so that each subject received each of the four mouthrinses only once for 1 week during the study. The subjects did not use any other oral hygiene procedure (including use of a toothbrush and dentifrice) during the 1-week period of time when they rinsed with their assigned mouthrinse product. The results indicated that use of the triclosan/copolymer mouthrinse provided a 49.8% reduction in supragingival plaque formation compared to the water placebo mouthrinse. The difference was statistically significant at the 99% level of confidence (P less than. 01). Similarly, the use of the triclosan/copolymer mouthrinse provided a 47.6% reduction in supragingival plaque formation compared to Plax antiplaque pre-brushing dental rinse. The difference was statistically significant at the 99% level of confidence (P less than. 01). The results further indicated that use of the triclosan/copolymer provided a 31.2% reduction in supragingival plaque formation compared to the alcohol placebo mouthrinse. The difference was statistically significant at the 99% level of confidence (P less than. 01).