Immunohistochemical study of osteonectin in various types of osteosarcoma. Polyclonal antibodies against osteonectin, a 32 kd non-collagenous bone protein, were applied for the histogenetic identification of variously differentiated osteosarcoma tissues. A strong positive reaction was found in matrix-producing osteosarcoma cells of the osteoblastic type, but pleomorphic or fibrosarcomatous osteosarcoma tissues reacted focally positive as well. Because the production of osteonectin depends on the osteoblastlike function of the individual tumor cell, a homogeneous immunocytochemical staining of all tumor cells cannot be expected. Nevertheless, the immunocytochemical demonstration of osteonectin in osteolytic tumors that produce no or scarcely any matrix seems to be a valuable tool for establishment of their osteogenic origin.