The author first considers the general conditions of reversibility, saturability, specificity and affinity which should be filled by all receptors, but emphasizes certain special difficulties on analysis of TSH receptors. TSH is a glycoprotein of molecular weight 28,000, composed of 2 alpha and beta sub-units; the role of each of these with regard to the receptor, is discussed. The TSH receptor is mainly localised in the outer part of the plasma membrane of the thyroid cells. An important point concerns the biological significance of the link between TSH and receptors and the activation of the adenylate cyclase AMPc system. One may experimentally dissociate with lecithinase these two cell events: treatment of thyroid tissue with this enzyme abolishes the biological response but does not affect the bond. The notion of hormone dependency of thyroid carcinoma is still classical but should be reconsidered in the light of new data concerning TSH receptors. Finally, the notion of variable receptivity of the thyroid tissue to TSH permits one to imagine another regulatory mechanism for which the receptors are responsible.