An immunoprecipitation assay was developed to determine the presence of antibodies against human TSH1 receptors. With this assay we were able to demonstrate that in comparison with sera from normal controls, 24 out of 30 (80%) sera from patients with untreated Graves' disease could immunoprecipitate more [125I]TSH-TSH receptor complexes. In 9 assays, an average of 14.1 ± 3.7% (sd) of the [125I]TSH-TSH receptor complexes was immunoprecipitated by the 30 Graves' sera vs 9.8 ± 3.0% by the normal pool serum (n = 23) (P < 0.001) and 7.7 ± 2.8% by the 22 normal sera (P < 0.001). One serum of the 24 positive Graves' sera was studied in detail. The results suggest that this serum contained an anti-TSH receptor auto-antibody directed towards a different determinant on the TSH receptor than the TSH binding site.