A rapamycin (RPM)-inducible fibroblast cell line expressing BMP2, BLK-RapBMP2, was previously developed using a stringent dimerizer-regulated transcription system to achieve more kinetic control of bone morphogenetic protein (BMP) expression for exogenous bone regeneration. This study examined the precise control of BMP2 synthesis and the induction of bone formation using various amounts of cells and rapamycin. The response to the rapamycin analogue (AP21967) caused the BLK-RapBMP2 cells to induce BMP2 expression in a cell amount-dependent manner corresponding to changes in the bone formation components in vitro and in vivo. The administration of rapamycin (1 mg/kg, i.p. for 6 weeks) induced variable ectopic bone formation to diverse number ($2-10{ imes}10^6$) of BLK-RapBMP2 cells in collagen hydrogel implants of a skin pouch in C57BL/6 mice. Microradiographic, biochemical (total calcium and phosphate concentration) and histological analyses suggest that control of the implanted cell number affects the level of rapamycin-induced bone formation. These results suggest that this technical control of BMP2 expression by adjusting the number of cells is a potential factor that might allow more precise control of bone regeneration.