Crystallization kinetics of hydroxyapatite, HAP, in the presence of acidic amino acids (aspartic and glutamic acid) were investigated at conditions of sustained supersaturation. In the presence of both aspartic and glutamic acid, the crystal growth rates of HAP decreased markedly. Their 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 affinity constants of both amino acids for HAP crystal surface were calculated. The apparent order of the crystal growth reaction was found to be equal to 2, suggesting a surface diffusion controlled mechanism.