The structural characteristics and glycoprotein nature of the human growth hormone (hGH) receptor in cultured lymphocytes (IM-9 cell line) were studied with the use of a bifunctional reagent (disuccinimidyl suberate) to couple 125I-hGH covalently to intact cells. After cross-linking, the hormone-receptor complexes were analysed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. A single band of Mr 140,000 was identified under reducing conditions. The labelling of this band was blocked by unlabelled hGH but not by insulin, ovine prolactin, bovine or ovine growth hormones. The Mr 140,000 band was immunoprecipitated by either anti-hGH antibody or by a monoclonal antibody against rat liver growth hormone receptor. In the absence of reductant two major bands of Mr 270,000 and 140,000 were found. On two-dimensional gel electrophoresis, with the first dimension in the absence of reductant and the second in its presence, the Mr 270,000 complex generated the Mr 140,000 band. The nature of the oligosaccharide chains of the receptor was studied by treatment with different glycosidases. The electrophoretic mobility of the Mr 140,000 receptor complex was markedly increased after digestion with endoglycosidase F but showed no or little change after digestion with endoglycosidase H. The Mr 140,000 band was also sensitive to neuraminidase treatment. In addition the 125I-hGH-receptor complex was adsorbed by immobilized wheat germ agglutinin and to a smaller extent by immobilized concanavalin A, lentil lectin, ricin I and ricin II. In conclusion, taking into account that hGH is a Mr 22,000 polypeptide, the binding subunit of the GH receptor in human IM-9 lymphocytes has an Mr of approx. 120,000. The native receptor may exist as a homodimer of the binding subunit formed by disulphide bonds. Furthermore, the GH receptor subunit contains asparagine N-linked type of oligosaccharide chains. Most, if not all, of these chains are of the complex type and appear to be sialylated whereas no high-mannose type chains are detectable in the mature form of the receptor.