Antibodies to 65Kd heat-shock protein (hsp) of mycobacterium leprae were measured by enzyme-linked immunosorbent assay (ELISA) in the three immunoglobulin classes in paired sera and synovial fluids of patients with rheumatoid arthritis (RA). Titers of anti-hsp antibody were expressed by optical density (OD) values for sera or indexes (OD values divided by amounts of immunoglobulin in each class) for synovial fluids and for their paired sera. Indexes of anti-hsp antibody were higher in synovial fluids than those in sera at 15/18 for IgG, 17/18 for IgA and 16/18 for IgM class. These results suggest the participation of anti-hsp antibodies to synovitis in RA.