The heterogeneous crystallization of octacalcium phosphate (OCP, Ca8H2(PO4)6.5H2O) on demineralized Type I collagen has been studied from metastable supersaturated solutions, at 37degreesC and pH=6.50, using the constant composition crystal growth technique. The induction period, before OCP crystal growth, varied markedly with the degree of supersaturation of the solution. The data obtained allowed us to determine the apparent order for the precipitation and the growth mechanism of OCP on Type I collagen. Infrared spectroscopy analyses indicated the progressive mineralization of collagen and observations by scanning electron microscopy confirmed the development of OCP crystals on the collagen surface. The influence of bovine serum albumin on both the kinetics of OCP nucleation and growth has also been investigated. Because this protein was adsorbed on calcium phosphate nuclei, it exhibited two distinct effects as a function of its concentration in solution. We proposed a mechanism explaining the interaction between albumin and calcium phosphate nuclei or crystals and its incidence on the OCP crystallization kinetics. Observations by scanning electron microscopy revealed a modification of the size and the appearance of crystals grown on collagen due to the adsorption of albumin on the crystal surface.