A highly specific binding protein for human growth hormone (hGH) has been isolated from human serum by hGH-affinity chromatography. A purification of approximately 1500-fold with a 30-40% recovery was obtained with essentially no alteration in binding characteristics. Covalent cross-linking of 125I-hGH to the binding protein, followed by analysis by SDS-polyacrylamide gel electrophoresis and autoradiography, revealed two specifically labeled complexes. Allowing for a 1:1 binding stoichiometry the binding proteins themselves had mean mol wts of 57,000 and 69,300. These increased slightly to mol wt 60,300 and 72,000 respectively in the presence of 100 mM dithiothreitol, suggesting the presence of intramolecular but not inter-subunit disulfide linkages. These data confirm the presence of the hGH binding protein(s) in human serum and define their gross structural nature.