The release of keratan sulphate (KS) bearing proteoglycan fragments from the extracellular matrix of cartilage into the synovial fluid is believed to be an early event in most joint pathologies. Quantitative analysis of KS in body fluids is therefore regarded as having a certain potential in monitoring articular cartilage catabolism. We describe the application of a non-competitive enzyme linked immunosorbent assay (ELISA) for the quantitation of KS-epitope in synovial fluids, using a monoclonal anti-KS antibody. Synovial fluids from 75 patients were analyzed, comprising the following disease groups: i) rheumatoid arthritis (n = 42), ii) osteoarthritis (n = 20), iii) gouty arthritis (n = 5), and iv) reactive arthritis (Reiter's disease, n = 8). Highest concentrations of synovial KS-epitope were found in reactive arthritis (median = 1410 ng/ml), and in gouty arthritis (median = 2105 ng/ml). However, significantly lower concentrations of KS-epitope (p less than 0.01) were observed in synovial fluids from patients with rheumatoid arthritis (median = 197 ng/ml) and osteoarthritis (median = 337 ng/ml). Although considerable variation of individual values was observed in all groups, a weak and inverse correlation between synovial levels of KS-epitope and inflammatory disease activity was seen only in patients with rheumatoid arthritis. However, KS-epitope levels did not correlate with either the synovial IL-1 activity, nor the number of synovial leucocytes.