A hydroxyapatite slurry was prepared by stirring into water a stoichiometric mixture of tricalcium phosphate and calcium hydroxide until the solid content was 50 wt%. Samples were then prepared by slip casting, drying and firing in vacuum at 1000 to 1300° C for 1 h. The fired samples were crushed and soaked in ordinary mineral water for up to 2 months and examined by differential thermal analysis and thermogravimetry, infrared spectroscopy, X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analysis, and direct-current plasma emission spectroscopy. One phosphate ion was found to be replaced by carbonate and practically all hydroxide by water, regardless of heat treatment. The infrared spectra showed a close resemblance to that obtained from natural bone.