Hashimoto's thyroiditis is an autoimmune disease in which autoantibodies reactive to a number of thyroid antigens are made. In order to investigate the autoantibody repertoire in this disease, B cells from four patients with Hashimoto's thyroiditis were immortalised and, after limiting dilution, screened for reactivity to thyroid antigens. After a second limiting dilution, one anti-thyroglobulin IgM-secreting clone from three patients, and four clones from one patient, were analysed. The Ig heavy and light chain genes from each clone were amplified using the polymerase chain reaction and sequenced. The resulting heavy and light chain sequences were heterogeneous, although the four clones from one patient and the clone from a second patient shared a germline VH sequence. All antibodies had similar functional affinity, comparable to serum IgG from Hashimoto's patients. The cross-reactivity of the antibodies was analysed against bovine and rat thyroglobulin, histones, cardiolipin and human skeletal muscle. The antibodies were polyreactive, indicating that they are probably natural autoantibodies of unknown pathogenic significance.