The effect of piroxicam on proteoglycan metabolism of human cartilage cells was investigated in two in vitro models. Cells or tissue samples were obtained from six different donors. Piroxicam levels used in the test systems ranged from 2 to 6 micrograms r/ml and were comparable with serum concentrations in humans after oral intake. Piroxicam increased the synthesis rates of proteoglycan in some batches of isolated and monolayer-cultured chondrocytes and in tissue-cultured articular cartilage. The fact that this increase in the synthesis of proteoglycan was restricted to some of the donors whereas isolated cells or tissue samples from other individuals remained unaffected illustrates the heterogeneity of different human donors. Depression of proteoglycan synthesis in the presence of the drug was not observed.