Although the efficacy of thein vivoosteogenic capabilities of synthetic octacalcium phosphate (OCP) crystal implantation can be explained through its stimulatory capacity for the differentiation of the host osteoblastic cell lineage, direct evidence that OCP supports bone regeneration by osteogenic cellsin vivohas not been shown. Mesenchymal stem cells (MSCs) isolated from 4-week-old male Wistar rat long bones were pre-incubated in osteogenic or maintenance medium in the presence or absence of basic fibroblast growth factor (bFGF). OCP/Collagen (OCP/Col) or collagen disks were seeded with MSCs that had been pre-incubated in osteogenic medium containing bFGF, which exhibited the highest differentiation induction, and then incubated for an additional day. The disks were implanted in critical-sized calvaria defects of 12-week-old male Wistar rats and the specimens were analysed radiographically, histologically, histomorphometrically, and by micro-computed tomography (CT) imaging at 4 and 8 weeks after the implantation. The OCP/Col·MSCs group rapidly induced more bone regeneration, even within 4 weeks, compared to the OCP/Col group without MSCs. The bone mineral density of the OCP/Col·MSCs group was also greater than the OCP/Col group. The Col·MSCs group did not exhibit prominent osteogenicity. These results indicate that OCP crystals in a collagen matrix efficiently promote exogenously introduced osteogenic cells to initiate bone regeneration if the cells are pre-treated in a suitable differentiation condition.