Proteoglycan subunits from human articular cartilage were fractionated by caesium sulphate density gradient centrifugation. A single heterogeneous population of molecules was produced whose average density decreased with increasing age of the individual from which they were obtained. At no density did the carbohydrate composition of any adult fraction resemble that of any newborn fraction, although there was considerable overlap in density. However, there was a similarity in amino acid composition between the most dense proteoglycans from the adult and those of least density from the newborn. The carbohydrate content of a 2-year-old proteoglycan was intermediate in composition, with high density fractions resembling the newborn and low density fractions resembling the adult. In addition, the proteoglycans of lowest density in both the newborn and two year preparations showed additional bands on agarose/polyacrylamide gel electrophoresis resembling the adult material. These results indicate that while a core protein of adult composition may occur in the juvenile proteoglycan it need not necessarily be glycosylated in an adult manner, suggesting that glycosylation is to some extent independent of the origin of core protein heterogeneity.