Point mutations of the donor splice site of intron 3 of the human GH-1 gene cause autosomal dominant inherited isolated growth hormone deficiency (IGHD II). The mechanism by which a defect in one GH-1 allele results in GH deficiency is obscure. Previously reported reverse transcription-nested PCR data suggested an overexpression of the mutant GH-1 allele. We employed alternative methods to determine the relative expression of mutant (C for G at +1 of intron 3) and normal GH-1 allele. The use of a second round PCR primer bridging exons 2 and 3 and specific for normal GH-1 messenger ribonucleic acid (mRNA) indicated equal quantities in mutant and control cells. Large scale messenger RNA extraction from Epstein-Barr virus-transformed lymphoblasts permitted assay by ribonuclease protection. In normal pituitary, there were three GH-1 mRNA species. The variant lacking exon 3 comprised 5% of the total GH-1 mRNA. The proband's lymphoblasts contained equal amounts of mRNA with and without exon 3. Only normal GH-1 mRNA was detected in controls. Secreted GH, measured by enzyme-linked immunosorbent assay was present in equal concentrations in media from normal and mutant cells. Thus, GH-1 mRNA lacking exon 3 was expressed in proportion to the dosage of the mutant gene, and dominant effects on GH secretion were not observed in lymphoblasts. These findings are compatible with a dominant negative mechanism involving interaction between normal and mutant proteins in secretory vesicles of somatotropes.