Basic fibroblast growth factor (bFGF) is a potent mitogen and acts as an autocrine/paracrine factor for osteoblasts. Long-term administration of bFGF in vivo increases osteoblast number and stimulates matrix formation, but induces hypophosphatemia and impairs matrix mineralization. The goal of this study was to examine the interaction between bFGF and low levels of organic phosphate in an effort to better understand the possible long-term therapeutic effects of bFGF. These data show that in vitro administration of bFGF accelerates the calcification process and lowers the phosphate threshold needed for successful bone nodule formation. This correlates well with the observed upregulation of mRNA production for alkaline phosphatase and osteocalcin at day 7. These findings help elucidate the mechanisms of bFGF action on bone marrow stromal cell differentiation and mineralization and indicate that the delay in mineralization observed in vivo may not be caused by decreased phosphate availability alone.