S-100 protein is an acidic calcium-binding protein that was originally isolated from the mammalian central nervous system in 1965. Initially, S-100 protein was thought to be specific to neuroectodermal tissues, but its presence in chondrocytes was recently reported. This study is an analysis of the distribution of S-100 protein in lesions of human cartilage and its possible significance. Several cartilaginous tumors, both benign and malignant, as well as normal epiphyseal growth plates, were examined for S-100 protein by the immunoperoxidase technique. Each cartilaginous lesion that was examined showed immunoreactivity for S-100 protein. The staining product was noted only intracellularly. The highest intensity of staining was seen in the hypertrophic chondrocytes of the zone of provisional calcification in the growth plate and in the large chondrocytes located adjacent to areas of matrix mineralization in cartilaginous tumors. In normal epiphyseal growth plates, the intensity of staining increased in chondrocyte cytoplasm as one moved from the proliferating columnar chondrocytes through the zone of hypertrophic chondrocytes to the hypertrophic, degenerating chondrocytes in the zone of provisional calcification. In cartilaginous tumors, the cells of enchondroma and of the cartilaginous cap of osteochondroma were more immunoreactive than those of chondromyxoid fibroma. In benign chondroblastoma, the chondroblasts were less reactive than the chondrocytes in areas of chondroid matrix production. The latter areas of chondroblastomas showed stronger immunoreactivity in the matrix-enclosed cells adjacent to areas of mineral deposition. Among conventional chondrosarcomas, grade-I tumors showed greater immunoreactivity of the chondrocyte cytoplasm than did those of a higher grade, in which chondroid matrix production was less abundant.(ABSTRACT TRUNCATED AT 250 WORDS)