B cells were directly cloned using EBV transformation fron the synovial tissue of patients with rheumatoid arthritis (RA), the objective being to investigate the B cell repertoire at the site of inflammation. The frequency of clones producing antibodies with rheumatoid factor (RF) activity was approximately 10% in those from the RA synovial tissue. Similar percentages of B cell clones from the peripheral blood of both RA patients and healthy controls also produced RF. However, almost all of these clones from the peripheral blood produced RF reactive not only with rabbit IgG or human IgG Fc but also with several other antigens (polyreactive). Only one of 654 clones (0.15%) from the RA peripheral blood produced RF specific to rabbit IgG or human IgG Fc (monoreactive). On the other hand, the frequency of clones producing monoreactive RF was approximately 30 times higher in RA synovial tissue. Furthermore, these B cells were activated in vivo to produce antibodies, since monoreactive RF was spontaneously produced from synovial tissue cells without the addition of B cell stimulators. No clones producing monoreactive RF were obtained from the synovial tissue of patients with osteoarthritis. These results suggest selective infiltration and/or proliferation of B cells committed to the production of monoreactive RF in RA synovial tissue.