Crystallization kinetics of hydroxyapatite, HAP, in the presence of Leucine, a natural amino acid with hydrophobic side groups, were investigated at conditions of sustained supersaturation, 37 degrees C, pH 7.40, ionic strength 0.15 M. In the presence of Leucine, the crystal growth rates of HAP decreased markedly. Tauhis action is due to adsorption and subsequent blocking of the active growth sites onto the surface of the HAP crystals. The kinetic results revealed that a Langmuir-type adsorption isotherm is followed and an affinity constants of k(aff )=20.26 x 10(2) L/mol for HAP crystal surface were calculated. The apparent order of the crystal growth reaction was found to be equal to two, suggesting a surface diffusion controlled mechanism.