The present study describes a double antibody technique to evaluate antithyroglobulin autoantibodies in human serum. Rabbit anti-human γG is used to precipitate the immunocomplexes between labelled thyroglobulin and autoantibodies. Thyroglobulin has been labelled either in vivo, or chemically by electrolysis of iodide, a procedure which also produced substantial dissociation of the protein. The double antibody technique was compared with haemagglutination of sheep red blood cells coated with human thyroglobulin. A good correlation was established between the tanned red cell haemagglutination titre and the double antibody technique when ‘ in vivo’ labelled thyroglobulin was employed. The use of chemically iodinated thyroglobulin, purified after labelling, increased the sensitivity of the test 100 fold so that 38% of sera from patients with thyroid diseases having negative haemagglutination titres precipitate between 15% and 65% of the labelled antigen. The sensitivity and simplicity of this method provide a useful tool in clinical as well as in experimental work.