The action of the fluoride ion has been investigated using calcium-deficient hydroxyapatites with compositions comparable with dental mineral. Whereas during carious dissolution there is a preferential loss of calcium by the mineral, the action of fluoride is to cause a preferential acquisition of calcium by the calcium-deficient mineral. The calcium: phosphorus ratio of material acquired by suspended hydroxyapatite on addition of fluoride was always in excess of the theoretical 1.67 for hydroxy- or fluorapatite and this is consistent with a selective uptake of calcium by the depleted mineral.