Regulation of serum insulin-like growth factors (IGFs), IGF binding proteins (IGFBPs), and growth hormone (GH) binding protein (GHBP) has been investigated in Ecuadorean patients with GH receptor dysfunction (GHRD) and in their heterozygous relatives (parents). Serum IGF-I and IGF-II levels were measured by radioimmunoassay (RIA). IGFBPs were identified by Western ligand blotting of serum samples, following separation by sodium dodecyl sulphate polyacrylamide gel electrophoresis, and relative quantification was performed with a scanning laser densitometer. Serum GHBP levels were measured with a ligand-mediated immunofunctional assay (LIFA) using a monoclonal antibody raised against the GHBP. These values were then compared with values obtained from normal, sex-matched Ecuadorean controls. Serum IGF-I, IGF-II, IGFBP-3 and GHBP concentrations were markedly reduced and serum IGFBP-2 values increased in the patients with GHRD. Serum IGF-I and IGF-II values were positively correlated in the patients with GHRD, but were not related to the age of the patient. IGFBP-2 and IGFBP-3 concentrations were inversely correlated in the patients with GHRD. When analysed by age, however, IGFBP-2 was related inversely and IGFBP-3 related positively to the age of the patient. The serum IGF-II levels of the male heterozygotes were significantly reduced when compared with sex-matched controls, but considerable overlap with normal values was found. No other biochemical indices were significantly altered in either male or female heterozygotes. Thus, although GHRD is characterized by dramatic reductions in serum levels of GHBP, IGF-I, IGF-II and IGFBP-3, none of these assays provides a reliable biochemical marker for heterozygosity.