The influence of antimicrobial mouthrinses containing dequalinium chloride or sanguinarine on early plaque formation was assessed in vivo in a clinical-experimental study. Rinses with water served as controls. After 24 and 72 hours, plastic films, which were applied to the buccal surfaces of six upper front teeth at the start of experiment, were removed and processed for transmission electron microscopic study. Dequalinium chloride or sanguinarine applied on plaque resulted in an increased and higher structured surface coating. Degenerated microorganisms were observed and the variety of bacteria seemed to be reduced. Both mouthrinses effected the early supragingival plaque formation. The effect of sanguinarine was more intensive than that of dequalinium chloride.