Hydroxyapatite (HAp) was synthesized in the presence of a variety of amino acids in order to investigate the effect of amino acid on the crystallinity and the solubility characteristics of HAp in the HAp-synthesizing condition. In the results of X-ray diffraction analysis. HAp synthesized in the presence of glycine (HAp-Gly). serine (HAp-Ser). aspartic acid (HAp-Asp) and glutamic acid (HAp-Glu) showed poor crystallinity compared with HAp synthesized in the absence of amino acid (HAp-con). The results of Fourier transform infrared spectroscope suggested the adsorption of these amino acids on HAp. Scanning electron microphotographs showed that the size and morphology of HAp adsorbed amino acids changed significantly. Furthermore, the solubility of these HAps increased significantly compared to HAp-con, each differing in four amino acids, However. other amino acids did not affect the crystallinity and morphology of HAp and had no significant change in their solubility. Collectively. this study suggests that the crystallinity and the solubility of synthesized HAp are different owing to the, variation of amino acids in the HAp synthesizing condition. It is expected that digestion-regulated HAp materials could be synthesized by using amino acid in their synthesizing condition. (C), 2002 Elsevier Science Ltd. All rights reserved.