Thyrotrophin (TSH) receptors have been extracted from human and porcine thyroid membranes by treatment with Triton X-100. 125I-Labelled bovine TSH was used to monitor receptor activity. Analysis by gel filtration and electrophoresis on acrylamide gels containing sodium dodecyl sulphate suggested that Triton extracts of human thyroid membranes contained TSH receptors with a molecular weight in the region of 50 000 closely associated with Triton micelles of approximate molecular weight 300 000. Isoelectric focusing studies indicated that the Triton-solubilized TSH binding activity had an isoelectric point of pH 4--4.5. The soluble TSH receptors were heat-labile, showed optimum TSH binding at pH 7.4 and reduced hormone binding at high ionic strength. The TSH binding characteristics of membrane-bound and solubilized human TSH receptors were similar and both preparations gave curved Scatchard plots. Solublized porcine TSH receptors appeared to have a similar molecular weight to the human receptors and were also closely associated with Triton micelles of approximate molecular weight 300 000. Scatchard analysis of TSH binding to membrane-bound or solubilized porcine TSH receptors gave approximately linear plots with association constants of 2.8 +/- 0.95 (S.E.M.) X 10(9) and 1.7 +/- 0.27 x 10(9) l/mol respectively. Comparison of the binding capacities of the solubilized and membrane-bound porcine receptors indicated that the 0.5% Triton extracts contained 40% of the original TSH binding activity and that this was present at a concentration of 25 ng/ml.