Immunoglobulin G (IgG) fractions prepared from the serum of 18 patients with primary myxedema, 9 patients with goitrous Hashimoto's thyroiditis, and 14 normal controls were tested for their ability to alter TSH stimulation of cAMP production in cultured human thyroid cells and the binding of TSH to its receptor. When compared with the cAMP increase induced by 0.1 mU/ml bovine TSH in the presence of normal IgG, cAMP accumulation was significantly inhibited (P less than 0.005) by the addition of IgG from patients with primary myxedema. TSH-induced cAMP accumulation was not affected by IgG from patients with goitrous thyroiditis. IgG from patients with primary myxedema also inhibited the cAMP increase induced by thyroid-stimulating immunoglobulins, but not against the increase induced by prostaglandin E1. None of the IgG tested affected the basal level of cAMP. Two potent inhibitory IgG were strongly positive for TSH-binding inhibitor immunoglobulins. Excluding these, no significant correlation was found between the thyroid stimulation-blocking activity and the TSH-binding inhibitory activity. These data suggest the presence of at least two different types of antibodies in primary myxedema which block adenylate cyclase stimulation by TSH and might be responsible for thyroid dysfunction and atrophy.