Abstract: Immunopathological changes of synovial tissue and fluid were investigated in patients with rheumatoid arthritis（RA）by direct immunofluorescent technique.Fluorescent immune complex（IC） deposits were detected in biopsied synovial tissue from all 7 patients with RA.IgG,IgM,IgA and C3 components were identified in the IC deposits in 6 cases.IgA was found to be negative in the remaining case.Sixteen specimens served as controls（12 from cases with other joint diseases rather than RA and 4 cases of sudden death）revealed IC deposits in none of them.Examination of synovial fluid revealed intracellular IC deposits in polymorphs in 28 of 29 RA patients（96.6%）. C3 and 1 to 3 of the above mentioned immunoglobulins were identified in the IC deposits.In contrast,in only 3 of 34 specimens from cases with non-rheumatoid joint diseases and in none of 11 normal subjects were IC deposits found in their synovial fluid cells.The results of the present study is consistent with the current concept that RA is an IC disease.Assay for ICs in the synovium and/or synovial fluid cells seem to be quite sensitive and may be useful in establishing the diagnosis of RA.