These experiments examined the unidirectional transfer of radioisotopically-labeled human placental lactogen (HPL) across term human amnion-chorion using in vitro techniques. Conditions of both equal and unequal concentrations of nonlabeled HPL were employed in the bathing solutions. All the data indicate that the bidirectional transfer of HPL can be accounted for by simple diffusion. There is no evidence of asymmetrical transfer, obligatory exchange diffusion, solvent drag, or a carrier mediated process. These results do not provide an explanation for the difference in the maternal plasma and amniotic fluid concentrations of HPL that exist in human gestation at term. Rather, a fetal metabolic role is postulated.