We investigated whether patients with rheumatoid arthritis have a polyclonal B cell activator (PBA) in their serum by using three methods: (1) the ability of any PBA to maintain the surface Ig of rabbit or human B cells in vitro; (2) the induction of blast transformation in human B lymphocyte cultures, and (3) stimulation of nude mouse spleen cells in vitro. All three methods indicated that a PBA is present in the serum of patients with rheumatoid arthritis but not in normal individuals or in patients with arthritis in which autoimmune phenomena have not been demonstrated. The entire PBA activity in rheumatoid arthritis patient serum was found associated with the macroglobulin fraction obtained by Sephadex G-200 chromatography and was precipitated by rabbit anti-human alpha 2-macroglobulin, but not by rabbit anti-Ig antibody. When alpha 2-macroglobulin was purified from patient serum the entire PBA activity was recovered in this fraction. Normal alpha 2-macroglobulin prepared by the same procedure had no PBA activity. Thus, the existence of a PBA associated with alpha 2-macroglobulin was demonstrated in serum of patients with rheumatoid arthritis.