Degradation of cartilage in osteoarthritis of man results in the release of sulphated glycosaminoglycans, particularly keratan sulphate, into tissue fluids. A study was made to evaluate these markers for osteoarthritis in the horse. Synovial fluid and serum levels of keratan sulphate, measured by an ELISA-inhibition technique, and sulphated glycosaminoglycans measured by specific dye binding assay, were found to be significantly increased (P less than 0.001) in joints from horses with osteoarthritis, compared with normal joints. Synovial fluids from joints with infective arthritis also showed high keratan sulphate levels, but statistically were not significantly different from osteoarthritis. Measurement of serum/synovial fluid levels of proteoglycan may enable cartilage degeneration to be detected and monitored and help more effective treatments to be developed in the equine species.