Mice immunized with soluble heterologous thyroglobulins developed autoantibody that cross-reacted with autologous thyroglobulin. There was a direct correlation between the temporal appearance and quantity of serum autoantibody and the presumed in situ formation of immune complexes in the interstitium of the thyroid glands. Immediately after the formation of interstitial immune complexes containing antibody of the IgG complement-fixing type, the thyroids were invaded by a transient but intense neutrophil infiltrate which within 1 wk was replaced by chronic mononuclear elements. By the combination of fluorescence microscopy and autoradiography, thyroglobulin was demonstrated to be one, if not the sole, antigen in the interstitial immune complexes. The interstitial immune complexes were granular to lumpy in appearance and formed at the basal area of the follicular cells in intimate association with the follicular basement membrane. Electron microscopy revealed electron dense deposits, presumably immune complexes, between the follicular basement membrane and the plasma membrane. The presumed in situ formation of immune complexes in this model is similar to that which occurs in the Arthus reaction and is a different mechanism of immune complex injury than that caused by tissue deposition of circulating immune complexes as occurs in serum sickness.