B cells are centrally involved as antigen-presenting cells in certain autoimmune diseases. To establish whether autoantibodies form immune complexes (IC) with self-antigens in autoimmune thyroid disease (AITD) and promote B cell uptake of self-antigen, sera from patients with Hashimoto's thyroiditis (HT), Graves' disease (GD) and healthy controls were incubated with human thyroglobulin (Tg) before adding normal peripheral blood mononuclear cells. The deposition of immunoglobulins and C3 fragments on B cells was then assessed. Inclusion of Tg in serum from HT patients promoted B cell capture of IgG and C3 fragments. Furthermore, the binding of Tg to B cells in preparations of normal blood cells was higher in HT serum than in serum from controls and correlated positively with the serum anti-Tg activity, as did the B and CD4+ T cell proliferation. Disruption of the three-dimensional structure of Tg by boiling reduced the proliferative responses. The data indicate that anti-Tg antibodies associated with AITD facilitate the formation of complement-activating Tg/anti-Tg complexes, binding of IC to B cells, and the subsequent proliferation of B and T cell subsets. This represents a novel mechanism for the maintenance of autoimmune processes in AITD and links autoreactive T cell responses with the presence of autoantibodies.