The remineralization of completely demineralized bovine dentin was studied in vitro by monitoring the velocities of fall of small particles through calcifying solutions. The density of a particle of dentin may be found from its velocity of fall through a fluid using Stokes' law. The minimum concentration product of calcium and acid phosphate ions of the solution in which remineralization would take place was 3.6 (mM)2 in the presence of 22 mM bicarbonate, pH 7.35, and ionic strength 0.1. This just above the solubility product of brushite (CaHPO4-2H2O). Incubation of decalcified dentin in a phosphoprotein removed from dentin during demineralization, or in phosvitin, had no effect on remineralization. The rates of remineralization and of the fraction remineralized were inversely proportional to particle size. This inverse correlation may be due to deposition of mineral in a surface layer of constant depth irrespective of particle size. The fraction of a particle remineralized was greatly increased by the use of highly supersaturated calcifying solutions or by the incorporation of fluoride into the solutions. The empirical reaction order of remineralization for both calcium and phosphate ions was found to be unity, which is, within the error limits, equal to the order of growth of seed crystals of hydroxyapatite in calcifying solutions of the same composition.