High performance liquid chromatography (HPLC) with a size exclusion column and ultraviolet monitoring was used to study the effect of synovial fluid hyaluronate concentration and degree of polymerisation on viscosity and mucin clot formation. Rotational viscometry measurements showed an exponential relation between the synovial fluid hyaluronate concentration and relative viscosity, the viscosity increasing particularly steeply with hyaluronate concentrations exceeding 2-2.5 mg/ml. The scattering of individual values observed around the expected curve was eliminated when both the hyaluronate concentration and its degree of polymerisation were taken into account. Hyaluronate concentration and degree of polymerisation also correlated with the quality of mucin clot, though only HPLC provided more detailed quantitative information about this association. Because HPLC allows reproducible and rapid simultaneous analysis of the synovial fluid hyaluronate concentration and the degree of polymerisation in small volumes of unprocessed samples it can be used in well equipped rheumatological units to replace other methods used previously.