There is as yet no plausible and convincing explanation for the etiology of rheumatoid arthritis. The authors investigated anti-synovial tissue antibody in the body fluid of rheumatoid arthritis by means of indirect immunofluorescent technic using non-affected synovial tissues as antigen. As the result the anti-synovial tissue antibody was detected in 7 cases of the 15 synovial fluid samples of rheumatoid arthritis and in two out of the six serum samples. The site of the localization of this antibody was demonstrated to be in the synovial membrane, especially in synovial cells and in the small blood vessel
walls situated immediately adjacent to the synovial surface, but it was found in no connective tissues other than synovial membrane. It seems that this anti-synovial tissue antibody should be considered as an independent factor from rheumatoid factor, and that rather than the rheumatoid factor it is more actively associated with the localization and progression of chronic inflammation within the rheumatoid arthritis joint.</p
