TSH-binding inhibitor immunoglobulins (TBII) have been detectedin patients with Graves’ disease and Hashimoto’s thyroiditis by using the radioreceptor assay of TSH. In untreated Graves’ patients, TBII levels correlated well with thyroidal^99mTc uptake at 30 min and the grade of epithelial hyperplasia of thyroid follicles. There were many Graves’ patients whose sera contained high TBII levels but no detectable bioassayable thyroid-stimulating activity (LATS), and in these patients, close correlation was observed between serum levels of TBII and bioassayable LATSprotector activity. TBII were detectable in 2 (10%) of 20 patients with Hashimoto’s thyroiditis, both of whom were clinically hypothyroid. The serum or IgG fraction from one of them, however, did not contain any significant LATS, LATS-protector, or humanthyroid adenylate cyclase-stimulating activity and caused inhibition of adenylate cyclase stimulation by TSH. In that patient, TBII may be acting to block TSH binding to TSH receptors, thuscausing TSH unresponsiveness and hypothyroidism.