Various tumors and tumor-like lesions of bone and cartilage were examined for S-100 protein using the avidine-biotin-peroxidase complex (ABC) immunostaining method. The most intense reactivity for S-100 protein was found in well-differentiated chondrocytes of enchondromas, osteochondromas and chondrosarcomas, and in normal epiphyseal cartilage. S-100 protein was positive in both polygonal stromal cells and chondrocytes of chondroblastomas and in chondrocytes of mesenchymal chondrosarcoma. In osteosarcomas not only chondroblastic areas but also osteoblastic areas showed positive cells. Reticulum histiocytic cells of eosinophilic granulomas and chordoma cells were positive for S-100 protein. The study yielded three main conclusions: (1) S-100 protein could be the marker for tumors of cartilaginous origin and differentiation, notochord origin, and T-zone histiocyte origin; (2) chondroblastoma can be distinguished from other histologically confusing giant cell lesions by using ABC to detect S-100 protein; and (3) S-100 protein has some relationship with tumoral calcification not only in cartilaginous tumors but also in osteosarcoma.