Bovine enamel and dentin specimens were overlaid with acidogenic Streptococcus mutans suspensions in agarose. In this model, the minimal demineralisation-inhibiting concentrations (MDIC) of hexetidine was determined in the presence of fluoride. A commercially available mouthwash containing 0.1% (2.9 mmol/l) hexetidine was diluted serially and added to the bacterial suspensions together with 0, 5.3, or 26.3 mumol/l fluoride (NaF). After 22 h of incubation at 37 degrees C the bacterial suspensions were removed and assessed for calcium and lactate. The results showed significant inhibitory effects of hexetidine on the demineralisation of the enamel specimens with a MDIC between 15 and 31 mumol/l hexetidine. In the presence of fluoride, approximately fourfold higher concentrations of hexetidine were needed for a significant additional protection of the enamel. No synergistic effect between hexetidine and fluoride was observed. For the demineralisation of the dentin specimens, the MDIC of hexetidine had a value between 31 and 61 mumol/l. At both these concentrations the dentin specimens were relatively less protected in the presence than in the absence of fluoride, and some synergistic effect between hexeditine and fluoride was observed.