Previous experiments have indicated that experimental autoimmune thyroiditis (EAT) induced in the Buffalo strain of rat by neonatal thymectomy closely resembles Hashimoto's thyroiditis. In these experiments we have analysed the serology of this type of EAT. Anti-thyroglobulin (Tg) antibodies were the only thyroid autoantibodies detected by immunoblotting. The IgG subclass distribution, analysed by a monoclonal anti-rat IgG subclass ELISA, altered during the course of disease. The proportion of IgG1 subclass Tg antibodies rose and the proportion of IgG2b subclass antibodies fell as disease progressed and the relative functional affinities of the anti-Tg IgG subclasses increased as disease progressed. The sera from animals with EAT were not cytotoxic in the presence of complement to a cultured rat thyroid cell line. In conclusion, EAT does not result in the production of antibodies against heterologous autoantigens as in Hashimoto's thyroiditis. Tg antibodies are produced which have similar properties to human Tg autoantibodies, including lack of cytotoxicity, and subclass restriction; these appear to have little pathogenic role in the disease.