Monoclonal antibodies TP-1 and TP-3, which detect an antigen expressed on the cells of human osteosarcoma, were tested by immunohistochemical staining for binding to normal and neoplastic canine tissues. In dogs, as in man, TP antibodies stained the tumor cells of all (13/13) osteosarcomas tested. Limited staining was found in a wide range of normal canine tissues. A proportion of cells in some canine carcinomas also stained, which was unexpected on the basis of the unique mesenchymal specificity of these antibodies in human tissues. TP monoclonal antibodies detect epitopes common to osteosarcomas in man and dog and are potentially valuable for the in vitro and in vivo comparative study of this tumor.