Content of microsomal antigen and thyroid peroxidase activity have been compared in preparations of thyroid microsomal membranes obtained by three different methods following thyroidectomy performed because of Graves' disease or nontoxic goiter. It was demonstrated that the microsomal membrane preparations originating from the thyroid tissue obtained following operation of goiter in Graves' disease are characterized by several times bigger content of microsomal antigen than those obtained after operation of nontoxic goiter. A sensitive ELISA method has been devised for the determination of microsomal autoantibodies in blood serum. The results obtained by this methods were found to be well correlated (r = 0.67) with those obtained with RIA kits manufactured by Serono. The level of antibodies in blood serum was determined in patients with thyroid disorders and the results were compared with those obtained in a group of patients with autoimmune diseases not associated with the thyroid and in healthy blood donors. The occurrence of antimicrosomal autoantibodies was demonstrated in 90.3% of patients with untreated Grave's disease, in 89% of patients with Hashimoto disease, and in 17.7% of patients with toxic goiter. Low titre of antimicrosomal autoantibodies was found in only small percentage of patients with nontoxic goiter (7.7%) and in patients with autoimmune diseases of nonthyroid origin.