A significant portion (about 12.7%) of healthy subjects was found to contain anti-thyroglobulin (anti-Tg) antibodies in their sera. We compared the binding activities of these antibodies and of anti-Tg autoantibodies from sera of patients with chronic thyroiditis with human thyroid peroxidase (TPO). The results obtained by ELISA indicated that out of 10 healthy subjects with anti-Tg antibodies, only four had anti-Tg antibodies capable of binding to TPO, whereas anti-Tg autoantibodies from almost all patients with chronic thyroiditis possessed high binding activities to TPO. By use of the immunoprecipitation method, it was also shown that although all anti-Tg autoantibodies from patients precipitated TPO, a majority of anti-Tg antibodies from healthy subjects could not precipitate TPO. Such findings cannot be ascribed to the differences in levels of anti-Tg autoantibodies and anti-TPO autoantibodies in sera and the differences in avidities of anti-Tg antibodies in sera between healthy subjects and patients with chronic thyroiditis. Thus, it can be concluded that anti-Tg antibodies from healthy subjects differ from those of patients with chronic thyroiditis with respect to TPO binding, probably due to difference in fine specificities of these anti-Tg antibodies.