Ten cases of dedifferentiated chondrosarcoma (DCS) were immunohistochemically and histochemically compared with 12 de novo malignant fibrous histiocytomas, 10 osteoblastic osteosarcomas, 9 conventional chondrosarcomas, and 4 fibrosarcomas (all of bone or soft tissues), in order to discern similarities and differences in the immunophenotypes of these neoplasms. All cases of DCS and malignant fibrous histiocytoma were reactive for alpha-1-antichymotrypsin, and several examples of both tumor types bound peanut agglutinin, and expressed positivity for alpha-1-antitrypsin and lysozyme. None of these four cellular markers was observed in de novo osteosarcoma and fibrosarcoma; in addition, conventional chondrosarcoma lacked all of them except for peanut agglutinin receptors. S100 protein reactivity and binding of wheat germ agglutinin were detectable in conventional chondrosarcomas and in rare cells of the anaplastic components of primary DCS, but not in malignant fibrous histiocytoma arising ab initio and the other sarcomas. These results suggest the evolution of a second neoplastic cellular clone in DCS, with primitive morphological and phenotypic characteristics. © 1987 Springer-Verlag.