Proteoglycan metabolism was investigated in tissue-cultured human cartilage. Normal cartilage obtained from a normal joint showed improving accumulation rates for 35S-labelled proteoglycans over a 3-6 weeks' period. The loss of newly synthesized molecules in the nutrient media was low and constant throughout culture. A decrease in accumulation of 35S-proteoglycan was observed in visually intact cartilage from joints showing foci of osteoarthrosis. This decrease was more pronounced in fibrillated cartilage. No clear effect on proteoglycan metabolism was observed when normal cartilage samples from the normal joint were incubated for several weeks in a nutrient medium containing niflumic acid. However, the cartilage samples from this donor tended to retain more proteoglycan aggregates in the intercellular matrix after a 6-weeks culture period in the presence of this NSAID. When niflumic acid was added to the incubation media of visually intact samples from pathological joints, significantly more newly synthesized proteoglycans were retained in the intercellular matrix. The effect was also observed and was more pronounced in the fibrillated cartilage samples.