The aim of this study was two-fold: firstly, to study the effect of high fluoride concentrations on carbohydrate metabolism in Streptococcus mutans present in biofilms on hydroxyapatite; and, secondly, to evaluate the effect of fluoride-bound hydroxyapatite on lactic acid formation in growing biofilms of Strep. mutans. Biofilms of a clinical strain of Strep. mutans on saliva-coated hydroxyapatite beads were incubated with sodium fluoride over a wide range of concentrations. At high fluoride concentrations (>10 mM) the incorporation of [14C]-labeled glucose decreased by 80-85%, at both pH 7.0 and 5.6. At lower fluoride concentrations, the effect of fluoride on the incorporation of labeled glucose was pH-dependent in both biofilm cells and in planktonic cells. At pH 7.0, fluoride at concentrations < 10 mM had little or no effect. Pretreatment of hydroxyapatite discs with fluoride varnish (Fluor Protector) or fluoride solutions caused a statistically significant reduction of lactic acid formation in associated, growing biofilms of Strep. mutans. Fluoride varnish and 0.2% (47.6 mM) sodium fluoride solution exhibited a statistically significant inhibitory effect on lactate production.