Chlorhexidine, and zinc in combination with triclosan, are used as anti-plaque agents in the prevention of gingivitis. The multifunctional activity of these compounds against bacterial cells has been proposed to include interference with sugar transport and reduction of glycolysis. In this study the ability of the agents to reduce acid production in dental plaque in vivo has been investigated. Samples of smooth-surface plaque were collected from individuals who had been rinsing for several weeks with (a) chlorhexidine (0.12%, Peridex); a combination of zinc and triclosan in mouthwashes containing (b) high and (c) low concentrations of humectant; or (d) a control mouthwash. Analyses using isotachophoresis showed that resting plaques in the chlorhexidine and zinc/triclosan groups contained less acetate than the control group. Acids were also measured 15 min after a glucose rinse. Compared with the control, the amount of lactate was significantly decreased (45%) in the chlorhexidine group, while lactate reduction (20%) in the zinc/triclosan (high humectant) group was not statistically significant.