A sandwich-binding protein assay to determine the concentration of hyaluronic acid (HA) in body fluids has been developed. In this method, a hyaluronic acid binding protein (HABP) was adsorbed to the surface of a solid phase, and HA bound to HABP on the solid phase was detected by biotin-conjugated HABP. The method could assay HA levels within 6 hours using precoated microwells with HABP. HA could be determined in the range of 2-500 micrograms/l by this method using 50 microliters of serum. Within-run precision (CV) was 5.2-10.2%. The specificity of HABP to HA was confirmed by the elimination of the reaction with treatment by hyaluronidase digestion. Serum HA levels (median; range) of patients with rheumatoid arthritis (34; 2-187 micrograms/l) were shown to be higher than those with osteoarthritis (1; 1-21 micrograms/ml) and healthy controls (2; 1-8 micrograms/ml). No correlation between levels of HA and rheumatoid factor was found. HA was demonstrated to be a potential diagnostic marker for rheumatoid arthritis, and this HABP assay could be useful for determination of HA in clinical laboratory tests.