In this paper we have summarized our findings on immune activity in patients with Rheumatoid Arthritis. RA is characterized not only by the formation of various autoantibodies but also of a hyperreactivity of the B cell system, shown as an increased DNA synthetic rate of blood non-T, non-monocytic lymphocytes as well as an increased number of actively antibody secreting cells both in the blood and the synovial fluid. Synovial fluid contains biological activity which synergizes with PWM for the induction of Ig-secreting cells in blood from healthy controls. The factor can also substitute for T cells in the PWM-induced antibody synthesis in vitro. This activity fits well with the finding that SF contains a factor which induces increased formation of IgG in LPS-pretreated mouse cell cultures. Experiments show that the factor leads to a preferential increase in the production of IgG2b antibody secreting cells. Therefore, we conclude that synovial fluid contains a B cell differentiating factor with a selective effect on the induction of a particular IgG subclass.