The one of striking microscopic feature is synovium hyperplasia in rheumatoid arthritis. The origin of the hyperplastic synovial lining cells in RA remains unsettled. Two marked monoclonal antibodies and two marked antisera are used to run the immunohistochemical study. Our finding revealed that synovial lining cells bear marks for blood monocytes. It is suggested that the increased synovial lining cells population appear to derive from bone marrow. Besides, the results of immunoelectron microscopy showed that synovial lining cells of type A and type C express HLA-DR antigen. HLA positive dendritic cells are distinctly more frequently seen in RA synovium than in that of osteoarthritis.