Immunohistochemical distribution of the Ca2+ binding proteins S100A1, S100A2, S100A4, S100A6, S100B, and bone morphogenetic protein (BMP) in chondro-osseous tumors and lesions, both benign and malignant, was investigated using specific anti S100 protein and BMP antibodies. Chondrogenic tumor cells of chondro-osseous lesions were characterized by the presence of marked staining for S100B and BMP, while they were only faintly reactive for S100A1, S100A2, S100A4 and S100A6. Dense fibrous connective tissue in osseous tumor and ossifying fibroma showed moderate immunoreactivity for S100A1, S100A4 and BMP. Immunoreactivity of S100A2, prominent in epidermal basal cells and associated or homologous cells of epidermal tumors or skin appendages tumors, was not found in cartilage and bone forming cells. Biological roles of S100B in chondroid cells may involve Ca2+-signaling in precalcified tissue, cytoskeletal protein organization and matrix formation since glycosaminoglycan synthesis is mediated by calcium ions. S100B positive cells in chondro-osseous structures also strongly expressed BMP. The present study allowed us to conclude that among the S100 proteins, the S100B in particular and S100A1, S100A4 and S100A6 but not S100A2 may be involved in the process of tumorigenesis of chondro-osseous tumors and BMP may have an important role in the chondroid and osseous differentiation. The detailed biological role of S100 proteins in chondro-osseous tumors is under investigation.