Urist, M. R. derronstratcd that a specific pi otein obtained fiom Lone matrix had osteogenesis, and named it as a Bone Morphogenetic Protein (BMP).In the present study, Urist method was employed to obtain crude BMP which was then implanted into mouse thigh muscle. Histological, enzyme histochemical, and immunohistochemical studies were employed to observe the osteoinductive process caused by BMP.Three days after implantation, a number of undifferentiated mesenchymal cells proliferated and the BMP implanted area was surrounded by the cells. Positive staining for laminin was found in the basement membrane of muscle fibers, and the cells adjacent to the basement membrane of myofibers also showed positive staining for laminin. At this time, both ALPase and ACPase were not active. From the electron micrographic features, dividing satellite cells and isolated mononucleate cells which migrated from myofiber were confirmed.Five days after implantation, spindle shaped undifferentiated mesenchymal cells became round in shape. Only ACPase reaction was found in the boadering area of BMP pellet.One week after implantation, undifferentiated mesenchymal cells in the proximity of BMP pellet developed into round shaped chondroblasts, and formed the chondro-matrix. Proliferating and resting chondroblasts were found by electron microscopy and a phagocytotic action by monocytotic cells was also found in the proximity of BMP pellet. At this time, both ACPase and ALPase were active.Two weeks after implantation, calcification of the chondro-matrix had started from a bordering area. Many phagocytes and chondroclasts appeared to reabsorb the cartilage.Three weeks after implantation, cartilagnous ossification further progressed and developed osteoid and bone marrow like tissues.Four weeks after implantation, bone remodering of a newly formed matrix existed with evidence of bone reabsorption.Six and eight weeks after implantation, bone reabsorption still continued.