24Background 25Circulating mesenchymal stem cells contribute to bone repair. Their incorporation in fracture 26 callus is correlated to their bioavailability. In addition, Granulocyte-colony stimulating factor 27 induces the release of vascular and mesenchymal progenitors. We hypothesized that this 28 glycoprotein stimulates fracture healing, and analyzed the effects of its administration at low 29 doses on bone healing. 30Methods 31 27 adult male Sprague-Dawley rats underwent mid-femur osteotomy stabilized by centromedullar 32 pinning. In a post (pre) operative group, rats were subcutaneously injected with 5µg/kg per day of 33Granulocyte-colony stimulating factor for 5 days after (before) surgery. In a control group, rats 34 were injected with saline solution for 5 days immediately after surgery. A radiographic 35 consolidation score was calculated. At day 35, femurs were studied histologically and underwent 36 biomechanical tests. 37