This study was undertaken to elucidate the effect of human interleukin-1 (IL-1) on the cartilage metabolism, with H235SO4 and 3H-proline as the respective indices for the metabolism of proteoglycan and collagen. We examined in vitro the 35S and 3H rates of incorporation after IL-1 was added to free chondrocytes incubated from human and porcine joint cartilage. As a result, IL-1 suppressed dose-dependently the secretion of both 35S and 3H into the supernatant of the medium containing free human or porcine chondrocytes. There was no difference between the control and the IL-1 group in the elution pattern of proteoglycan on the column-chromatogram. These results indicated that human IL-1 did not affect the molecular weight of the proteoglycan produced by human and porcine chondrocytes.