The impact of osteonectin for differential diagnosis of osteogenic bone tumors: an immunohistochemical and in situ hybridization approach. Thirty-three osteosarcomas at various grades of histologic differentiation, including chondroblastic, osteoblastic, and fibroblastic variants, were investigated immunohistochemically for evidence of osteonectin. Twenty-two cases of varying types of osteosarcoma were examined with in situ hybridization for mRNA expression of osteonectin. Immunohistochemically, osteonectin was present in all the osteosarcomas in this study. With in situ hybridization, 12 out of 22 osteosarcomas showed a positive signal. Two osteochondrosarcomas, seven chondrosarcomas, and one mesenchymal chondrosarcoma were also studied with regard to the localization of osteonectin, either immunohistochemically or by in situ hybridization. Immunohistochemically, osteonectin was present in all the chondroid lesions except for one osteochondroma. However, in situ hybridization of osteonectin mRNA was negative in all the chondroid lesions we studied. This study revealed that immunohistochemical localization of osteonectin is not useful in providing conclusive diagnosis of osteosarcoma. In situ hybridization of osteonectin mRNA might be useful in differentiating osteosarcoma from nonsteogenic bone tumors.