A modification of a colorimetric assay was used to determine synovial fluid total and individual sulphated-glycosaminoglycan concentration in various clinical presentations of joint disease in horses. Concentrations of synovial fluid and serum sulphated-glycosaminoglycan (GAG) were measured by the 1,9-dimethylmethylene blue (DMMB) dye assay in normal horses (n = 49), horses with acute (n = 26) or chronic (n = 27) joint disease (defined by clinical, radiographic, and clinicopathological parameters), and horses with cartilaginous lesions at diagnostic arthroscopy, but with normal radiographs and synovial fluid (n = 9). Horses with acute joint disease were subdivided into moderate acute (n = 21) and severe acute (n = 5) joint disease on the basis of synovial fluid analysis and clinical examination. Horses with chronic joint disease were subdivided into mild chronic (n = 9), moderate chronic (n = 10), and severe chronic (n = 8) joint disease on the basis of synovial fluid analysis, clinical examination, and radiographic findings. The concentrations of chondroitin sulphate (CS) and keratan sulphate (KS) were analyzed in each sample following sequential enzymatic digestion of the sample with chondroitinase or keratanase. In addition, the concentration of hyaluronate (HA) in each sample was determined by a colorimetric assay following digestion of the sample with microbial hyaluronidase.(ABSTRACT TRUNCATED AT 250 WORDS)