Addition of the amino acids serine, tyrosine and 4-hydroxyproline with polar uncharged side groups at physiological pH levels to a supersaturated calcium phosphate solution reduced the crystal growth rate of hydroxyapatite (HAP) at 37 degrees C. Depending on the side group of the amino acid different inhibiting activities were observed. The reduction was also found to be related to the additive concentration following an expression suggested by the kinetic Langmuir-type adsorption model. This relationship implies that I-IAP crystallization kinetics was affected through blockage of the active growth sites on the crystal surface. In all cases the apparent order of the crystal growth reaction was found to be n = 2, suggesting a surface-diffusion-controlled spiral growth mechanism.