The receptors for TSH have been studied in human thyroid tissue to assess their density and binding characteristics in various disease states. A single set of similar independent receptors appeared to be present in both healthy and pathological thyroid tissue. Their binding affinity for bovine TSH averaged 1.1 X 10(10) l/mol in healthy tissue and, with the exception of papillary carcinoma which showed some reduction, was not significantly altered in the various disease states studied. No receptors with low binding affinity were found. The number of receptors was significantly greater in toxic diffuse goitre and in hyperfunctioning follicular adenoma (but these tissues came from patients given antithyroid drugs and often iodine preoperatively), and was reduced in Hashimoto's thyroiditis. In well-differentiated thyroid carcinoma, the number of receptors was similar to or greater than in normal tissue, but in undifferentiated and medullary carcinoma, and in lymphoma of the thyroid, receptors were completely absent. Tracer-binding data obtained with human TSH were uniformly lower than the corresponding data obtained with bovine TSH, but showed an analogous pattern of differences amongst the various normal and pathological tissues.