Triclosan has been incorporated into toothpaste to enhance inhibitory effects on bacterial metabolism in dental plaque. Many studies have confirmed these effects by showing a reduction of accumulation of dental plaque, gingivitis and calculus. However, there is no evidence for triclosan having an inhibitory effect on the dental plaque-induced demineralization of the dental hard tissues. Therefore, the effect of 0.3% triclosan added to non-fluoride and fluoride toothpaste was tested in an in vitro model, in which bovine enamel specimens were to be demineralized by acids produced in overlaying Streptococcus mutans suspensions. In a first set of experiments the toothpastes were added to the S. mutans suspensions at 1:100, 1:1000 and 1:10,000 (w/v) dilutions. After 22 h incubation at 37 degrees C the suspensions were removed and assessed for calcium and lactate content, and pH. In this set of experiments, triclosan had no additive protective effect to the non-fluoride or fluoride toothpaste. In a second set of experiments, the enamel specimens were immersed daily for 3 min in 30% (w/v) slurries of the toothpastes before the 22 h incubation with the S. mutans suspensions. Under these conditions, triclosan showed an additional protective effect compared with non-fluoride toothpaste at a low concentration of S. mutans cells (0.07 mg cells dry weight per 600 microL suspension). It is concluded that the enamel surface may act as a reservoir for triclosan, which may protect the enamel surface against a mild acid attack. In combination with fluoride, however, as in toothpaste, triclosan has no additional protective effect against demineralization.