Proteoglycans synthesized by human cartilage were studied in explants and in chondrocyte cultures. The D1 fraction proteoglycans radiolabelled with 35SO4 were analyzed by 3-16% sodium dodecyl sulfate polyacrylamide gel electrophoresis or on 3-5% acrylamide gels. After 24 h in culture, osteoarthritic and age matched nonarthritic cartilage synthesized one aggrecan core protein (LI) whose apparent size after chondroitinase ABC/keratanase digestion was approximately 520 kDa. Three proteoglycan core proteins were found in the D1 fraction of the medium compartment of nonarthritic chondrocytes, whose apparent sizes were approximately 520 kDa (LI), approximately 390 kDa (LII), and approximately 480 kDa (LIII). The LII and LIII core proteins were routinely absent from osteoarthritic chondrocytes. Chondrocytes from a 28-year-old patient with osteoarthritis secondary to congenital hip dysplasia synthesized principally the LI proteoglycan core protein.