The crystal growth of hydroxyapatite (HAP), the most stable calcium phosphate phase at 37 degreesC, pH 7.40, and ionic strength 0.15 M, in the presence of four amino acids with uncharged polar side groups (glycine, cysteine, cystine, and glutamine) has been investigated by the constant composition technique: Crystallization took place exclusively on well-characterized HAP crystals in supersaturated solutions only with respect to this calcium phosphate salt. The kinetic results indicated a surface diffusion mechanism. The presence of the amino acids inhibited the crystal growth of HAP possibly through adsorption onto the active growth sites of the HAP crystal surface. Cysteine in particular caused a reduction in the crystal growth of HAP up to 81%. The kinetic results favored a Langmuir type adsorption model, and the value of the affinity constant for each amino acid was calculated. Each value was related to the nature of the amino acid side group.