The precipitation kinetics of hydroxyapatite from calcium phosphate aggregates was investigated by the continuous-angle laser light- scattering technique in the systems CaCl2-H3PO4-H2O and KCl-CaCl2H3PO4-H2O at 25 degreesC. The apparent molecular weights of aggregates increased with time, and precipitation of hydroxyapatite occurred in both systems. However, the radius of gyration of the aggregate was almost constant in the former system, while it increased with the increase of the molecular weight in the latter system. Fractal dimension analysis revealed that loosely distributed particles in the aggregate transformed to a three-dimensional closely packed structure during the precipitation of hydroxyapatite in the former system, while the aggregate and the crystalline particles within it increased in size with the increased density and act as growth centers of hydroxyapatite in the latter system.