Recently, bioactive agents to stimulate bone formation have been available in the orthopedic field. We have shown previously that a single, local injection of basic fibroblast growth factor (bFGF) contributes to the formation of a larger cartilage (soft callus) but does not promote replacement of the cartilage by osseous tissue during experimental closed femoral fracture healing. Aiming at a clinical application, the present study was undertaken to clarify the effects of locally injected bFGF on bone (hard callus) formation and the mechanical properties of the callus in closed fracture healing in rats. Immediately after fracture, a carrier (200 muL of fibrin gel) containing 100 mug of bFGF or carrier alone was applied to the fracture site. At days 42 and 56 postfracture, the bone union rate, bone mineral density (BMD), and mechanical properties (strength and stiffness) of the callus were evaluated. Unexpectedly, with the exception of reduced stiffness in the FGF-injected callus at day 56, none of these parameters showed a significant difference between the control and the FGF-injected groups. Furthermore, the temporal expression pattern of OPN mRNA during healing was very similar between groups. We conclude that, in the healing of closed fractures of long bones, administration of bFGF forms a larger callus but does not necessarily accelerate the healing process.