Keratan sulphate is an integral component of the large aggregating proteoglycans of mature human articular cartilage. The keratan sulphate content of chondrocytic proteoglycans increases during maturation, and it is a useful marker of mature-type chondrocytic proteoglycans. Ordinarily, in cell culture, chondrocytes from non-neoplastic tissues dedifferentiate, diminish or cease to synthesize aggregating proteoglycans with the same amount of keratan sulphate as those formed in vivo, and do not maintain their in vivo phenotype. In tissue culture, this down-regulation of synthesis of keratan sulphate is irreversible. The study of the metabolism of mature human chondrocytes has been hampered by the absence of stable models. We report a cell-line, 105KC, derived from a human chondrosarcoma, that has maintained a stable proteoglycan phenotype during more than three years of culture. Analysis with immunofluorescence suggested that 105KC cells continued to synthesize keratan sulphate in long-term culture. Biochemical analysis demonstrated that 105KC cells maintained the production of chondrocytic large-aggregating proteoglycans and that keratan sulphate composed 13 per cent of their glycosaminoglycan content. To our knowledge, 105KC represents the first model to have maintained the post-fetal chondrocytic proteoglycan phenotype in stable culture. This study documents the feasibility of the development of mature chondrocytic cell-lines and sheds light on the biological characteristics of chondrosarcomas.