We assayed the production of an IgG autoantibody, anti-thyroglobulin (anti-Tg), in vitro by peripheral blood lymphocytes from patients with autoimmune thyroid diseases. We subfractionated B cell populations and tested them for anti-Tg production with excess numbers of T cells in the presence of pokeweed mitogen. When lymphocytes with surface membrane IgM were depleted by treatment with anti-mu and complement, the remaining mu cells, ineffective in forming IgM, were capable of producing anti-Tg, although less well than untreated cells. Separation of B cells into IgM- and IgM+ fractions after rosetting with anti-mu coated ox red blood cells produced an IgM- fraction capable of making decreased amounts of anti-Tg, while the IgM+ fraction was inactive. We found no evidence for IgG+ precursor cells for anti-Tg formation. We conclude that the long-lived memory cells responsible for anti-Tg production in vitro are not typical IgM-bearing cells. They may lack IgM or carry a form of IgM that is poorly bound by anti-mu.