The present study was designed to investigate whether octacalcium phosphate (OCP)-precipitated alginate (Alg) promotes osteoblastic cell proliferation and bone regeneration in comparison with Alg itself. Alg, known to lack mammal cell attachment capability, was used as the matrix to test the distribution effect of OCP in a three-dimensional environment. A series of Alg/OCP scaffolds with different pore sizes was prepared by centrifuging Alg gels precipitated by OCP crystals. The scaffolds had a bimodal distribution of pores (ultrafine pores: approximately 100 nm; relatively large pores: from 6.0 to 51.7 microm) and over 86% porosity. The osteoconductive capability of Alg/OCP was determined by examining mouse bone marrow stromal ST-2 cell proliferation after 3 days in vitro and bone regeneration in mouse calvaria critical-sized defect after 21 days. The analyses showed that ST-2 cell proliferation and bone regeneration increased with an increase in the pore size and reached the highest level in the 51.7 microm pore scaffold. The results suggest that OCP-precipitated Alg provides a better scaffold for osteoblasts to attach and proliferate in a three-dimensional environment and promotes bone regeneration, indicating that OCP is a candidate material to modify the surface of non-cell-interactive polymeric scaffolds, such as Alg, into an osteogenic condition.