Newly synthesized and endogenous proteoglycans of human osteoarthritic knee cartilages were studied in short term explant culture. There were more newly synthesized proteoglycans than endogenous uronic acid containing proteoglycans found in the A1 fraction generated by associative CsCl density gradient centrifugation. Thirty-five percent of the proteoglycans in the A1 fraction were shown to elute in the void volume on Sepharose CL-2B, under associative conditions. Reduction and alkylation of the A1 fraction indicated that the endogenous proteoglycan monomer was hydrodynamically smaller than the newly synthesized macromolecule. This was confirmed by a Sepharose CL-2B chromatography of the A1D1 fraction. In comparison to endogenous proteoglycans, the newly synthesized proteoglycan monomers were less capable of forming stable aggregates with exogenous hyaluronic acid. We also found a statistically significant decrease (p less than 0.05) in the amount of already existing proteoglycan subclass when compared to the amount of newly synthesized proteoglycans in the most dense A1D1 fraction. By contrast, a statistically significant increase (p less than 0.05) in the amount of endogenous vs newly synthesized proteoglycans was seen in the A1D2 fraction but not in fractions of a lower buoyant density.