Although relatively small amounts of zinc and magnesium ions are known to be incorporated into tooth enamel and bone minerals, they may have a major influence on the mineralization kinetics. In this study, the results of equilibrium adsorption of these ions on hydroxyapatite (HAP) and electrophoretic mobility measurements suggest that at solution concentrations of zinc ion above about 1 ppm, the formation of zinc phosphate (Hopeite) dominates the surface properties. In the presence of zinc and magnesium ions, the constant composition growth of HAP is markedly reduced and the results suggest a Langmuir-type adsorption of these additives at active growth sites on the HAP crystals. Zinc ion is one of the most effective of the simple metal ions in inhibiting HAP crystal growth.