We present another patient with the rare disorder of isolated GH deficiency (IGHD) type 1A. The diagnosis was confirmed by the clinical course, serology and genetic structure. Analysis of the post GH treatment serum of this patient with several thoroughly characterized monoclonal antibodies against GH established the presence of polyclonal, high titre and high avidity IgG-class antibodies against GH. These had the ability to neutralize GH as they could inhibit the binding of radiolabelled GH to GH-receptors on IM9 lymphoblastoid cells. DNA restriction mapping indicated a deletion of the GH-N gene and a family DNA pattern that was consistent with the proposed autosomal recessive aetiology of this disorder. These findings explain the inability of this patient to synthesize GH and his total immunological intolerance to GH replacement therapy.