Bone mineral of human is different in composition from the stoichiometric hydroxyapatite (Ca-10 (PO4)(6)(OH)(2)) in that it contains additional ions, of which CO3 2-is the most abundant species. Carbonate-substituted hydroxyapatite (CHA) bodies were prepared by the hydrothermal treatment of highly porous calcium carbonate (CaCO3) body at 120 degrees C in 1M M2HPO4 and M3PO4 solutions (M = NH4 or K). It was found that CaCO3 body was almost transformed into CHA body after hydrothermal treatment for 24 h irrespective of type of phosphate solution. However, a small amount of CaCO3 still remained after the treatment in K3PO4 for 48 h. Crystal shape of CHA bodies prepared in those solutions except for K2HPO4 was flake-like, which was different from that (stick-like) of original CaCO3 body used for the preparation of CHA body. CHA prepared in the K2HPO4 showed globule-like crystal. Average pore size and hole size of the CHA bodies were 150, 70 mu m and their porosities were about 89% irrespective of the solution. Carbonate content was slightly higher in the CHA bodies obtained from potassium phosphate solutions than in those obtained from ammonium phosphate solutions. Mostly B-type CHA was obtained after the hydrothermal treatment in the potassium phosphate solutions. On the other hand, mixed A-and B-type CHA (ca. 1-2 in molar ratio) was obtained in the ammonium phosphate solutions. The content of CO3 2-in the CHA body depended on the type of phosphate solution and was slightly larger in the potassium phosphate solutions.