In this study we report the spontaneous production of autoimmune anti-thyroglobulin antibodies (Tg Ab) by peripheral blood lymphocytes from patients with autoimmune thyroid disease and the use of these cells to study the regulation of this production in vitro. Using a mitogen free microculture system we have found a significant correlation between an individuals' serum titre and the amount of Tg Ab produced in vitro. At low B:T cell ratios Tg Ab production decreased. Specific depletion of suppressor T cells and helper T cells was accomplished using the monoclonal antibodies OKT 8 and OKT 4 respectively. Suppressor cell depletions resulted in increases in Tg Ab production while depletion of helper cells had no consistent effect. The addition of pokeweed mitogen had no stimulatory effect on spontaneous thyroglobulin antibody production. In contrast, when T cell conditioned media was added to the cells, Tg Ab production increased substantially in all patient cultures tested but not in normal control cultures. The results of this study are consistent with the existence of functional thyroid-antigen specific suppressor cells in the peripheral blood which are actively involved in the regulation of autoantibody producing B cells. The results obtained with the cultures maintained in conditioned media show that autoimmune lymphocytes are extremely sensitive to one or more stimulatory factors present in the media and suggests an important role for the production of soluble lymphocyte factors and the expression of receptors for these factors in the aetiology of autoimmune thyroid disease.