 The presence of plaque on tooth surfaces as a predictor of chlorhexidine side effects has not been evaluated especially because most studies utilized a previous disruption of biofilms in all dental surfaces before the beginning of rinsing. The purpose of this study is to compare staining and calculus formation after 0.12 percent chlorhexidine between previously plaque-free and plaque-covered surfaces by means of an experimental gingivitis model. 