The kinetics of crystallization of hydroxyapatite on elastin has been studied using the constant composition method. The apparent order for the HAP crystallization reaction was found to be 2.0 +/- 0.2, suggesting a surface diffusion controlled mechanism. Analysis of the initial rates of the reaction as a function of the solution supersaturation, according to the classical nucleation theory, yielded a value of 104 +/- 13 mJ m-2 for the surface energy of the growing phase and a three-ion cluster forming the critical nucleus. The adsorption of inorganic phosphate and calcium by the elastin has also been investigated as a function of concentration, pH, and ionic strength. Phosphate is taken up extensively by the elastin surfaces, and the dependence of adsorption upon the ionic strength of the medium suggests an appreciable contribution from elastostatic forces. Calcium ions were not taken up by elastin from solutions of calcium chloride up to 5 X 10(-3) mol dm-3.