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The IgG transporter responsible for ferrying maternal IgG across the human placenta to fetal circulation has not been identified, although the human homologue of the neonatal rat Fc receptor (FcRn), a heterodimer with pH-dependent IgG affinity, structurally similar to MHC Class I molecules, was recently proposed as a candidate. Affirming this hypothesis, we describe herein the specific copurification from human placenta of 46- and 14-kDa proteins by IgG affinity at acid pH. The larger protein, characterized by its amino acid sequence and by immunoblot, is the alpha-chain of human FcRn (hFcRn). The smaller is beta2-microglobulin. Their coisolation by ligand affinity suggests that they comprise the hFcRn heterodimer. Placenta sections stained immunohistochemically with anti-hFcRn alpha-chain peptide Abs show extensive expression of hFcRn in the syncytiotrophoblast and traces in the endothelium and …