The function of HLA-DR positive thyrocytes on thyroid autoantibody production has been examined to test the hypothesis that such HLA-DR positive thyrocytes may initiate or aggravate autoimmune thyroid disease. Thyrocytes were cultured (precultured) with leucoagglutinin (which stimulated thyrocyte expression of HLA-DR, beta 2-microglobulin (beta 2-m) and thyroid microsomal antigens) and then cocultured with peripheral blood mononuclear cells. Thyroid antibody production by the latter was then measured. There was no evidence of induction or enhancement of thyroid-microsomal and thyroglobulin autoantibody production in supernatants from the cocultures of autologous peripheral blood mononuclear cells and HLA-DR positive thyrocytes from normal controls and patients with Graves' disease. Furthermore, stimulation of B lymphocytes from patients with autoimmune thyroid disease with a combination of Staphylococcus aureus Cowan I plus supernatants from autologous cocultures of peripheral blood mononuclear cells and HLA-DR positive thyrocytes from normal controls and Graves' disease, produced significantly less microsomal antibody and thyroglobulin antibody than similar cocultures with HLA-DR negative thyrocytes, although total immunoglobulin G (IgG) was similar in both groups. The effect of supernatants from allogeneic cocultures on microsomal antibody thyroglobulin antibody and total IgG production was no different between HLA-DR positive and HLA-DR negative thyrocytes. These data suggest that HLA-DR positive thyrocytes may have a protective role against thyroid autoimmunity rather than a pathogenic role for it.