The HLA-DR antigen was investigated in synovial epithelia of rheumatoid arthritis, osteoarthritis and normal traumatic joints, using monoclonal anti-human HLA-DR antibody and horseradish peroxidase-conjugated antibody. The HLA-DR staining was observed in an electron microscope. HLA-DR antigen was observed to be present on the surface of both macrophage-like (type A) and fibroblast-like (type B) cells in synovial epithelia in all rheumatoid arthritis, osteoarthritis and normal joints. Since type A and B cells are ultrastructurally considered to be of synovial origin, the findings suggest that the expression of HLA-DR antigen on the surface is one of the common attributes of type A and B cells in synovial epithelia, even before cellular infiltration of chronic inflammation in rheumatoid arthritis. These cells may play an important role in the initiation of rheumatoid inflammation, since HLA-DR antigen is considered equivalent to murine Ia antigen.