The nature of the TSH receptor in adenoma and carcinoma of the thyroid gland was studied using a radioreceptor assay technique. A membrane fraction of tissue homogenate was obtained by discontinuous sucrose gradient ultracentrifugation, and 125I-TSH, labelled by a lactoperoxidase method, was purified with a receptor adsorption method. Both the capacities and the association constants of high affinity receptors (4 x 10(9) M-1) and of low affinity receptors (0.073 x 10(9) M-1) observed in the normal thyroid were almost identical to those of the thyroid of Graves' disease and those of thyroid adenoma. Although the two papillary carcinomas examined were found to have two kinds of TSH receptors, one of the carcinomas showed decreased association constants for both high affinity and low affinity receptors.