The neonatal Fc receptor (FcRn) is named for the source from which it was first purified, the intestinal epithelium of neonatal rats. The human homologue of FcRn was first found in the placenta, where is presumably involved in transfer of maternal IgG to the fetus. FcRn has since been found in capillary endothelium, intestinal epithelium and other tissues. More recent data has indicated that FcRn has a function beyond the transfer of maternal IgG--the maintenance of constant serum IgG levels. This review summarizes the status of our knowledge on the structure, expression and function of the neonatal Fc gamma receptor.