In the present work we employed fibrin in order to assess its capability to induce biological mineralization. Fibrin is a very important factor in the blood-clotting system. Structurally, fibrin is an ordered organic matrix which has a periodic structure that repeats every 230 Angstrom. Hydroxyapatite, HAP and octacalcium phosphate (OCP) are the most interesting calcium phosphate salts. Hydroxyapatite is thermodynamically the most stable calcium phosphate which is mostly used as a model compound for the study of biological-calcification processes. On the other hand, octacalcium phosphate has been proposed as a precursor of hydroxyapatite whose formation is favoured kinetically in solutions supersaturated to both salts. The kinetics of crystallization of HAP and OCP on fibrin were studied using the constant composition technique. The onset of HAP crystallization started immediately after introducing the substrate in the supersaturated solution. Unlike HAP crystallization induction periods were observed before the appearance of OCP precipitate in a solution supersaturated with respect to both HAP and OCP. Using nucleation rate equations derived from the classical homogeneous nucleation theory, interfacial energies and the size of the critical nucleus for both HAP and OCP were calculated. Phosphate was taken up extensively by the biological molecule studied. The dependence of adsorption upon ionic strength and pH of the medium suggests an appreciable contribution of electrostatic forces. Controversially calcium ions did not exhibit any detectable adsorption from solutions containing calcium dichloride at concentrations ranging from 1 x 10(-4) to 5 x 10(-3) M in 0.15 M NaCl supporting electrolyte, 37 degrees C, pH = 7.4. From the results above it follows that formation of HAP on fibrin may be initiated via adsorption of inorganic phosphate on the biological substrate.