Joint fluid levels of carboxy-terminal type II procollagen peptide (pCOL II-C) were measured in osteoarthritis, rheumatoid arthritis and traumatic arthritis by a newly developed one-step enzyme immunoassay (EIA). The detection limit of the new method was as low as 0.2 ng/ml. The levels of pCOL II-C were significantly (P < 0.001) higher in osteoarthritis and traumatic arthritis than in rheumatoid arthritis. In osteoarthritis, pCOL II-C levels were higher in moderately afflicted patients. Since type II collagen is a unique component of cartilage, pCOL II-C levels in joint fluids could reflect the synthetic activity of type II collagen of chondrocytes in the diseased joint and therefore could be utilized as a simple marker of type II collagen synthesis in articular cartilage in joint diseases.