Formulating drinks with reduced erosive potential is one approach for reducing dental erosion. In this study, whole casein was added to citric acid solutions representative of soft drinks, and the hydroxyapatite dissolution rate was assessed. Adding 0.02% (w/v) casein to acid solutions significantly reduced the hydroxyapatite dissolution rate by 51 +/- 4% at pH values of 2.80, 3.00, 3.20, 3.40, and 3.60, although the baseline dissolution rates of course varied as a function of pH. The protein concentration [0.002, 0.02, and 0.2% (w/v) casein] had no significant effect on dissolution inhibition. Adding both casein and calcium to citric acid resulted in a further reduction in the dissolution rate at low and intermediate calcium concentrations (5 and 10 mM) but not at higher calcium concentrations (20 and 50 mM). Ionic strength had no significant impact on the efficacy of casein. Casein also significantly reduced the hydroxyapatite dissolution rate when the hydroxyapatite was coated with a salivary pellicle. The reduction in dissolution rate is ascribed to firmly adsorbed casein on the hydroxyapatite surface, which stabilizes the crystal surface and inhibits ion detachment.