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The kinetics of growth of calcium phosphate on synthetic HAP seed crystals has been studied at 37 degrees C and at pH values of 4.5 and 5.0 by the pH-stat controlled addition of base. Following an induction, DCPD crystal growth takes place with growth kinetics characteristic of the crystallization of pure DCPD seed. The effect of stirring rate, HAP seed concentration and initial degree of supersaturation with respect to DCPD on the kinetics of the growth process have been determined. The crystallization appears to be controlled by a surface reaction and nucleation of DCPD on the HAP substrate is completed in the initial induction period. Formation of DCPD under these conditions closely simulates plaque and calculus production in the mouth. Thus calculus that develops from the initial plaque is known to contain an appreciable amount of DCPD.