A mouthrinse containing 0.1% (w/v) of the isoquinoline alkaloid sanguinarine chloride with 1,000 ppm Zn was tested in a model with sucrose-induced plaque formation. Rinsing twice daily for 1 min with 10 ml of the rinse resulted in a mean plaque index of 1.39 compared to 1.81 for placebo without zinc and sanguinarine. Chlorhexidine (0.1%) resulted in a plaque index of 0.23. All groups were significantly different at the p < 0.05 level (t test). A single rinse with 0.05% (w/v) sanguinarine chloride without Zn did not affect the in vivo acidogenicity of established plaque beyond 10 min after the rinse. Antiplaque effects of sanguinarine chloride up to 0.1% (w/v) against sucrose-stimulated plaque in vivo could not be demonstrated in the present study.