: We studied the possibility of standardizing the radioreceptor assay for the detection of anti-thyrotrophin receptor antibodies, also called thyrotrophinbinding inhibitor immunoglobulins (TBII), to circumvent the problem of inter-assay variability in TBII index calculations. We developed a procedure for conversion of the thyrotrophin-binding inhibition caused by standard amounts (10 mg IgG/ml) of 1.6 m ammonium sulphate precipitates into thyrotrophin-equivalents, i.e. the amount of thyrotrophin required to cause the same thyrotrophin-binding inhibition. We were able to determine a normal range for thyrotrophin-binding inhibition which exhibited little inter-assay variability and was applicable to individual radioreceptor assays. The normal range, expressed in thyrotrophin-equivalents or the logarithm of μU thyrotrophin, was 2.616–3.645, which corresponds to 413–4420 μU. The thyrotrophin-binding inhibition by 1.6 m ammonium sulphate precipitates from sera of 17 untreated patients with Graves' disease was also expressed in thyrotrophin-equivalents. For 15 patients (88%) the thyrotrophin-equivalent values were above the normal range, whereas only 10 patients (59%) had a positive TBII index (using the same sample). This indicated that 5 patients had anti-thyrotrophin receptor antibodies in spite of their negative TBII index. Standardization based on the IgG content of the test samples therefore seems to be imperative. Pig serum 1.6 m ammonium sulphate precipitates, which were tested in a porcine assay system, yielded thyrotrophin-equivalent values that corresponded closely to the normal range for the human assay system, suggesting that the normal range of thyrotrophin-binding inhibition is in the same order of magnitude in porcine and human assay systems.