Octacalcium phosphate (OCP) has been advocated to be a precursor of biological apatite crystals in bones and teeth. Our previous studies showed that synthetic OCP stimulates bone regeneration, followed by the progressive conversion of OCP into hydroxyapatite (HA), when implanted in bone defects. However, the precise mechanism to induce the osteogenic phenotype in osteoblasts by OCP has not been identified. The present study was designed to investigate whether the physicochemical aspect, specific to and derived from the structural properties of OCP, influences the function of an osteoblastic cell line, mouse bone marrow stromal ST-2 cells. Different amounts of synthetic OCP and synthetic sintered ceramic HA were coated onto 48-well tissue culture plates. The amounts of OCP and HA were controlled to strengthen their intrinsic physicochemical properties, in which the milieu around the crystals will be modified during the culture. The roughness of the OCP coatings was independent of the amount of coating. Chemical analyses of the supernatants of the OCP coatings revealed that the concentration of Ca2+ decreased with increasing amounts of OCP, while the concentration of inorganic phosphate increased markedly, most probably through OCP--apatite conversion. ST-2 cells were cultured on the OCP or HA coatings up to day 21. The OCP coating caused a significant decrease in cell attachment and in the initial stage of proliferation, dependent upon the amount of coating. On the other hand, OCP enhanced the expression of osteogenic markers, including type I collagen, alkaline phosphatase, and osterix. However, HA did not alter the expression of these markers in ST-2 cells cultured on different amounts of HA coating. These results demonstrated that OCP is capable of inducing the differentiation of stromal cells into osteoblastic cells, especially differentiation into early stage osteoblastic cells, prior to reaching the stage of mature osteoblastic cell lineage.