The relationship between bone morphogenetic protein and neoplastic bone diseases. The monoclonal antibody against bovine bone morphogenetic protein was used for demonstration of bone morphogenetic protein (BMP) in neoplastic bone diseases. The avidin-biotin-peroxidase complex method demonstrated that BMP mainly exists in the cytoplasm of tumor cells of osteosarcoma and chondrosarcoma. Immunostaining showed that a majority of osteosarcomas and all of the chondrosarcoma cells contained a large quantity of BMP. Conversely, none of the fibrosarcomas showed positive staining. Thus, it was possible to differentiate osteosarcomas from fibrosarcomas by immunostaining. In fibrous dysplasia of bone, BMP was abundant in the fibrocellular tissue that had osteogenic activity. In contrast, fibrous tissue of ossifying fibroma showed weak positive staining; only the osteoblasts rimming the bone showed a positive reaction. Immunostaining showed that BMP was also detected in other neoplastic bone diseases such as osteoma, chondroma, and other tumors. 