In order to measure TSH receptor antibodies (TRAb) we tried to set up a radioreceptor assay using human thyroid membranes. Due to lack of appropriate binding activity of the material obtained, we decided to use a kit which provides solubilized porcine membrane-receptors to TSH instead of human membranes, as well as calibrators that have been standarized in a receptor assay against MRC LATS std B. With these reactives we have measured TRAb in sera from 7 normal controls (C), 54 thyrotoxic patients (43 diffuse goiters [BDH], 8 multinodular goiters [BHM] and 3 Subacute Thyroiditis [TSA]), 3 patients with Hashimoto's Thyroiditis (TH) and in 6 non-hyperthyroid Graves ophtalmopathy patients. Measurement were initially performed using calibrators and the results expressed as U/L; since a very good correlation between the expression U/L and the calculated Inhibition Index (I.I.) was found (r = 0.99, n = 15, p < 0,001), results are shown using latter. In C mean +/- SD value for I.I. was 3.4 +/- 2.37%, so we decided to use, as cut off criteria for differentiating between normal and abnormal results, the figure 11%, which represents the mean +/- 3 SD. According to this, 93% of BDH had elevated TRAb activity while only slightly more than one third of MBH had elevated values, this difference being highly significant (p < 0,0001); both TSA and TH patients showed low TRAb activity while all Graves ophtalmopathy pts had elevated values, thus suggesting that they had a latent disease. We concluded that the methodology that is adequate and practical for clinical purposes. Our results show that measurement of TRAb activity is useful in establishing the etiology of hyperthyroidism in an individual patient so we feel that it should be of common use in most, if not in all, thyrotoxic patient. Also it provides help in the differential diagnosis of patients with exoftalmus of unknown etiology. Its usefulness remains to be proved in the follow-up of BDH pts after been treated with antithyroid drugs.