AbstractThe objective of the present investigation was to evaluate to what extent mouthrinses containing triclosan and chlorhexidine may modify the amount of de novo plaque that forms on tooth surfaces adjacent to healthy and inflamed gingival units. 10 volunteers were recruited. On day 0, gingival crevicular fluid (GCF) was obtained at predetermined sites and gingivitis (GI) was assessed. A careful oral profylaxis was given to each of the volunteers who subsequently abstained from all mechanical plaque control measures for the following 18 days. During the first 4 days (rinse phase I), they rinsed with either 0.12% chlorhexidine. 0.06% triclosan or placebo solution. Clinical examinations (GCF, GI) were repeated and the amount of plaque formed determined on days 4, 7 and 14. On day 14, the participants received a new professional tooth cleaning after which rinse phase II was initiated. During this 2nd phase, the participants rinsed for 4 days with the same mouthwash preparation and in the same manner as during rinse phase I. The examinations were repeated on day 18. Each participant received a comprehensive oral prophylaxisis and was instructed to perform meticulous mechanical plaque control during the following 4 weeks. A 2nd experimental period was then initiated. A total of 3 experimental periods were repeated until all subjects had rinsed with the 3 different mouthwash preparations. The results demonstrated (i) that significantly more plaque formed at sites with gingivitis than at surfaces adjacent to healthy gingival units and (ii) pre‐existing gingivitis significantly increased the amount of de novo plaque that formed in subjects who rinsed with mouthwash preparations containing chlorhexidine and triclosa