Individual proteins (transferrin, ceruloplasmin, alpha 2-macroglobulin, IgG, IgA and IgM) were examined in blood and synovial fluid of 11 patients with Reiter's syndrome. ARA-1981 was used as a diagnostic criteria. The control group included 40 patients with rheumatoid arthritis and 31 with osteoarthrosis of the knee joint. Statistically significant differences between the concentrations of individual proteins in patients with Reiter's syndrome, rheumatoid arthritis and osteoarthritis were only established in synovial fluid for IgM and for IgM index Csf/CS.