Particulate fractions of guinea-pig thyroid homogenate contained a single class of receptors which bound 125I-labelled bovine thyrotrophin (TSH) by a saturable, reversible process with an affinity constant of 2 \ m=x\ 109 1/mol. The binding process was specific for TSH, and corresponded with the activation of adenylate cyclase. Cleavage of hormone\p=m-\receptor bonds by treatment with lyotropic agents resulted in the release of unchanged labelled TSH. The radioligand receptor assay system was sensitive to 0\m=.\015mu. TSH. Bovine or mouse thyroid showed reduced binding affinity with correspondingly reduced sensitivity.

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