In Graves' disease (GD), an antireceptor autoantibody disease, individual variability in the pathogenic interaction between TSH receptors and autoantibodies has been reported. This variability can be due to allotypic (person to person) variability in the receptors or differences in autoantibody amount or specificity. This fundamental issue was investigated by evaluating immunoglobulin G (Ig)-induced TSH receptor modulation in thyroid tissue from 19 patients with GD. TSH receptor modulation by Graves' Ig was defined as the appearance of 1 class of high affinity binding sites, instead of the usual 2 classes of binding sites. Ig-induced modulation of receptors occurred in 9 of 19 (47%) experiments with autologous (patient's own) tissues and correlated with the presence of TSH receptor antibodies, measured as TSH binding inhibitor Igs. Of these 9 receptor-modulating Graves' Ig preparations, 7 (78%) also had a receptor-modulating effect in other patient's (homologous) thyroid tissue. Nine of the 10 Graves' Ig preparations that were negative for TSH receptor-modulating activity in autologous thyroid tissue were tested with other patients' thyroid tissues; 7 (78%) were negative, and all were TSH binding inhibitor Ig negative. We conclude that variability in the occurrence of TSH receptor modulation was associated with the presence or absence of TSH-binding inhibitor Ig. No evidence for allotypic differences in TSH receptors in GD was found.