Obese strain chickens develop severe spontaneous autoimmune thyroiditis several weeks after hatching, characterized by mononuclear cell infiltration and antibodies to thyroglobulin (Tg). The presence of antibodies to Tg suggests that Tg is an important antigen in this disease, but it does not provide definitive evidence. To clarify this point, Obese strain chicks were tolerized at hatching with Tg and then examined up to 6 wk later for antibodies to Tg, thyroid pathology, and function. Various tolerance regimens were tested. The optimal conditions were i.v. injection of Tg within 24 hr of hatching, and injection of at least 1 mg. Tg isolated from normal thyroid glands was satisfactory, and it did not have to be deaggregated. Tolerance induced by the above procedure significantly retarded all parameters of autoimmunity, although by 6 wk of age some of the tolerized chicks had severe thyroiditis. Multiple weekly injections of Tg were no more effective than a single injection at hatching. Interestingly, a single injection at hatching was very effective, yet it was cleared from the circulation within 24 hr. In summary, tolerance induced with Tg had a profound effect on the disease and thus provides good evidence for the role of Tg in this disease.