Fibrin, a protein endoproduct of blood coagulation was found to be a substrate favoring the deposition of vaterite crystals from stable supersaturated solutions at pH 8.5 and at 25 degrees C. The apparent order for the vaterite crystallization reaction was found to be 1.0 +/-0.1, suggesting a surface diffusion-controlled mechanism. The crystallization was studied by the constant solution composition technique thus making it possible for relatively large amounts of the overgrowth phase to be formed and identified exclusively as vaterite. Fibrin stabilizes this calcium carbonate polymorph, preventing the transformation to the thermodynamically more stable calcite. 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 21 +/- 1 mJ m(-2) for the surface energy of the growing phase and a three-ion cluster forming the critical nucleus.