Fetal growth is thought to be independent of the concentration of GH, although circulating levels of GH are high in the human fetus. To elucidate the role of GH in fetal development, levels of GH-binding protein (GHBP) were measured in the serum of nonpregnant and pregnant women and neonates as well as in amniotic fluid obtained at various stages of gestation. Total GHBP (the sum of free GHBP and GHBP bound to GH) is measured by a ligand-mediated immunofunctional assay. GHBP concentrations in adult serum were not changed by pregnancy or the stage of gestation. A significant correlation was observed between the concentration of GHBP in the umbilical artery and vein. No correlations were observed between the GHBP concentration and such measures of fetal growth as fetal weight and fetal age. Although the neonatal concentrations of GHBP were significantly lower than those of pregnant women, no correlation was observed between them. GHBP was also present in the amniotic fluid from early to late gestation at concentrations higher than in the cord serum of the neonate. The amniotic GHBP concentration in late gestation was significantly higher than in early gestation. GHBP appears to be derived from GH receptors of fetal organs (most probably fetal liver). The low level of GHBP in fetal serum may be the result of a decrease in GH receptors caused by high levels of circulating GH. GHBP levels in amniotic fluids may be related to the development or maturation of the fetus.