Four natural amino acids with hydrophobic nonpolar side group (alanine, phenylalanine, proline, and methionine) were examined and their activity on hydroxyapatite, HAP, crystallization was assessed. The method used was the constant composition technique, at low supersaturation levels. Crystallization took place exclusively on well-characterized HAP seed crystals introduced in supersaturated solutions only with respect to the calcium phosphate salt. The fit of the experimental data to the Langmuir adsorption isotherm indicates an inhibition mechanism based upon molecular adsorption of the amino acids on the HAP crystal surface at active growth sites. The inhibitory effect was related to the adsorption affinity constant of each amino acid to the crystal surface, which depends on the nature of the amino acids' side group.