The thyrotropin (TSH) receptor is an important but poorly characterised component of the thyroid cell surface [l] and in this paper we describe an analysis of the receptor using affinity labelling [2]. Detergent solubilised porcine and human TSH receptors have been partially purified by Sepharose -TSH affinity chromatography [3] and then covalently crosslinked to 125 I-labelled TSH. Analysis of the TSH-TSH receptor complexes by sodium dodecylsulphate polyacrylamide gel electrophoresis (SDS-PAGE) suggests that both porcine and human TSH receptors contain a similar basic unit of two peptide chains linked by one or more disulphide bridges. These basic units bind one molecule of TSH and have relative molecular masses (M,) of 87 000 in the case of the porcine TSH receptor and 100 000 in the case of the human TSH receptor.