The effect of phospho-L-serine in supersaturated solutions of calcium phosphate was investigated at plethostatic conditions. Phospho-L-serine inhibited the crystal growth of hydroxyapatite {Ca5OH(PO4)(3), HAP} by blocking the crystal growth centers through adsorption. Complete inhibition was attained with a phospho-L-serine concentration of 4 x 10(-4) M. Analysis of the kinetics and adsorption isotherm results showed a Langmuir-type adsorption. The amount of phospho-L-serine adsorbed on MAP decreased as pH increased from 7.4 to 10.4. The adsorbed species of phospho-L-serine, which in the pH range 7.4-10.4 are mainly the monoprotonated species (HL2-), are located in the inner Helmholtz plane (IHP) of the double layer at the HAP/electrolyte interface, interacting with the surface of HAP with both electrostatic and chemical interactions. The location of the negatively charged HL2- species at the IHP caused a shift of the zeta -potential to more negative values. Finally, a model was established for the adsorption of phospho-L-serine onto the HAP surface at pH 7.4, according to which an ion pair forms between the protonated amino group of one HL2- species and one negative site of the surface of HAP, i.e., the phosphate or hydroxyl group. Due to electrostatic repulsion, the negatively charged deprotonated carboxyl and phosphate groups of the adsorbed HL2- species are oriented in a manner ensuring the maximum possible distance from the negatively charged surface of MAP.