The usefulness of solubilized TSH-receptor for measuring TSH and TSH-displacing antibodies (TDA) in a radioligand receptor assay (RRA) was investigated. Crude thyroid membranes were solubilized with Triton X-100. A soluble protein fraction was isolated by DEAE-Sephadex A-50 ion-exchange chromatography. The binding of 125I-TSH to the solubilized fraction was studied. It was maximal after 45 min at 37 C, whereas at 4 C a similar binding was observed after at least 17 h. The binding was inhibited by Ca2+ and Mg2+ with concentrations higher than 1 mM and by NaCl in the concentration range 17-150 mM. No inhibition of binding was found with normal serum up to 100 microliter. In contrast, 1.5 microliter of a Graves' disease serum showed significant inhibition. Using the system to measure TSH in serum, as low as 10 microunits TSH could be estimated. Compared to an RRA using thyroid membrane fractions, the sensitivity for TSH in serum as well as for TDA is increased at least tenfold in our system.