The effect of bone matrix protein of osteonectin on de novo formation of apatite was studied in a wide range of calcium phosphate solutions in the presence of collagen. In every solution, from which amorphous calcium phosphate, octacalcium phosphate, or apatite precipitated as a possible initial phase, osteonectin at concentrations less than 1 microM retarded the precipitation, subsequent transformation to apatite, and ripening crystal growth of apatite. Collagen present as either reconstituted or denatured form had no effect on the osteonectin-associated reactions as well as osteonectin-free reactions, and no structural correlation was observed between collagen fibrils and any of the calcium phosphates that appeared in our system. Direct measurement of free calcium levels in the solutions suggested that the reduction in calcium activity due to complexing with osteonectin hardly explained the inhibitory activity of osteonectin in retarding the formation of apatite. Instead, our transmission electron microscopic (TEM) observation strongly suggested that the primary mechanism for osteonectin to inhibit the formation of apatite is to block growth sites of calcium phosphates nucleated. The apatite thus formed in the presence of osteonectin showed less resolved X-ray diffraction patterns, partly because of smaller crystallites as suggested by TEM.