The acquisition of maternal antibodies is critical for immunologic defense of the newborn. In humans, maternal IgG is actively transported across the placenta. The receptor responsible for this transport has not been identified definitively. We report the isolation from a placental cDNA library of clones encoding the alpha-chain of an immunoglobulin G (IgG)-Fc receptor (hFcRn) that resembles a class I major histocompatibility complex antigen. The DNA and predicted amino acid sequences are very similar to those of the neonatal rat and mouse intestinal Fc receptors, rFcRn and mFcRn. These receptors mediate transport of maternal IgG from milk to the blood-stream of the suckling rat or mouse. Like rat and mouse FcRn, hFcRn binds IgG preferentially at low pH, which may imply that IgG binds hFcRn in an acidic intracellular compartment during transport across the placenta.