Mice of strains high and low responders to thyroglobulin were immunized with mouse thyroglobulin emulsified in Freund's complete adjuvant. Groups of mice were killed at weekly intervals and the serum thyroxine concentration was measured with a solid-phase RIA while the titre of thyroglobulin antibodies was determined by passive haemagglutination and the magnitude of thyroid infiltration with mononuclear cells was scored. In other groups of mice, similarly immunized, radioactive iodine uptake was measured at various times after immunization. In almost all mice the lowest level of thyroxine and the lowest radioiodine uptake were observed 2 weeks after immunization. There was no clear relationship between the thyroid function and the titre of thyroglobulin antibodies or the extent of the cellular infiltrates in the thyroid.