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Thyroid glands of Obese strain (OS) chickens with spontaneous hereditary autoimmune thyroiditis were studied by direct immunofluorescence (DIF) with an anti-chicken immunoglobulin–FITC conjugate for local immunoglobulin (Ig) production in plasma cells and germinal centres. Many plasma cells and most of the germinal centres showed positive staining in DIF. The Ig property of this stained material was verified by DIF blocking and indirect immunofluorescence (IIF) tests with specific unlabelled anti-chicken Ig sera. In a chronological DIF study of thyroid glands from OS chickens aged 1–18 weeks, Ig-producing plasma cells could be already detected in the 1st week of life. DIF tests with TRITC-labelled chicken thyroglobulin revealed positive staining of plasma cells with preferential localization in close proximity to, or even between, follicular epithelial cells, suggesting the anti-thyroglobulin autoantibody nature of at least some of the locally produced Ig.