The 'high-affinity' growth hormone binding protein (GHBP) corresponds to the extracellular domain of the membrane-associated GH receptor. In man, it is produced by proteolytic cleavage of the extracellular portion of the receptor, while in the rat and mouse, it is the product of alternative splicing of GH receptor mRNA. Because of the relationship between the GHBP and the GH receptor, measurement of serum concentrations of GHBP provide an index of GH receptor concentrations and/or affinity.