Growth hormone (GH) exerts multiple actions and GH receptors have been demonstrated in a variety of tissues. Changes in the number of hepatic GH receptors have been demonstrated in several models of growth failure in rats. Cloning of the rabbit and human liver GH receptor has shown that the receptor is a single polypeptide chain of 620 amino acids, made of an extracellular hormone-binding domain, with 7 cysteines and 5 potential glycosylation sites, a unique transmembrane domain and a long cytoplasmic domain. The GH receptor belongs to a new family including prolactin and cytokine receptors. Signal transduction pathways are unknown for these receptors, which do not possess any consensus sequences homologous to tyrosine kinases. The GH-binding protein (GH-BP), identified in plasma of man and other species, corresponds to the extracellular binding domain of the membrane GH receptor. Evaluation of the GH-BP is a direct approach to the GH receptor in man in vivo. Complete absence of GH binding activity has been found in the plasma of patients with Laron dwarfism, in particular those for whom a mutation in the GH receptor gene has been demonstrated.