In two patients with growth hormone (GH) insensitivity syndrome (Laron syndrome), in whom the GH receptor is able to bind the hormone, the D152H mutation was identified, and lack of dimerization was proposed to explain GH resistance in these patients. To examine further the consequences of the substitution of conserved aspartate 152 on the function of the GH receptor (GHR), we reproduced the mutation in vitro on the full length GH receptor cDNA from man and rat. Effects of the mutation on expression and activity of the GHR were analyzed in 293 cells transfected with wild-type and mutant GHR cDNAs. Mutant human receptor protein was expressed at a lower level than wild-type receptor and its activity was reduced: GH-dependent signal transducer and activator of transcription 5 (Stat5)-mediated transactivation of a reporter gene was lower in 293 cells transfected with mutant GHR cDNA than in transfected cells expressing a comparable level of wild-type GHR. The membrane-bound form of the mutant and of the wild-type human GHR were able to homodimerize, as suggested by the size of the complexes detected in cross-linking experiments with 125I-human (h) GH, and also by the activity in the functional test. With the soluble GHR resulting from proteolysis of the wild-type membrane form, no dimeric complexes could be detected. However, when a soluble receptor lacking the transmembrane and cytoplasmic domains of the receptor was expressed, wild-type and not mutant GH binding protein (GHBP) was able to form dimers in the presence of hGH. The amino acid substitution has no effect on either expression or function of the rat receptor. Structural modeling of D152H soluble human and rat GHR (GHBP) supports the species-specific functional consequences of the mutation. Evaluation of the functional importance of the mutation strongly suggests that impairment in expression and activity of the mutant receptor, rather than complete lack of dimerization, explains the GH resistance of the patients.