A monoclonal antibody raised against bone morphogenetic protein （BMP-McAb） has been used to demonstrate the presence of bone morphogenetic protein（BMP） in experimental fracture healing. Rabbit mandibles were fractured using standardized methods and left to heal for 3

7

14

21 and 24 d

respectively. The avidin-biotin complex （ABC） method demonstrated an accumulation of positively stained primitive mesenchymal cells at the fracture site in the hematoma stage of bone repair. These cells appeared to undergo differentiation into positively-stained chondroblasts and osteoblasts during the phase of callus formation. Undifferentiated mesenchymal cells showed a high positive reactivity in the early post-fracture stages but a much lower reactivity during the remodelling phase.The results of our study suggest that bone inductive processes are accompanied by the presence of BMP in osteoprogenitor cells during fracture healing of the mandible and that BMP may play a significant role in osteogenesis during bone healing.