Immunopathological studies were carried out on 117 patients with chronic thyroiditis. Circulating antibodies to thyroglobulin were demonstrated by tanned red cell hemagglutination (TRC), agar gel precipitation (PT), passive cutaneous anaphlaxis (PCA) and Praunitz-Kustner's Reaction (PKR). Serum antibodies to a cytoplasmic component of thyroid epithelial cells were demonstrated by complement fixation test (CF), cytotoxic factor (tissue culture) and fluorescent antibody technique. Antibodies to a second colloidal antigen and an antinuclear factor were demonstrated by fluorescent antibody technique. The incidence of thyroid autoantibodies in the sera of patients with chronic thyroiditis was 11.1% for PT, 65.0% for TRC, 41.9% for PCA, 44.4% for PKR, 50.0% for CF, 55.6% for cytotoxic factor, 100% for cytoplasmic factor, 100% for CA_2 antibody, and 7.8% for anti-nuclear factor. Thus, all the patients had least one positive test for antibody detection.