Objective: Human placental isoferritin, which is composed of a 43-kd protein subunit, is exclusively reactive with the CM-H9 monoclonal antibody. The p43 exerts immunosuppressive activity during pregnancy. The aim of this study was to localize the expression of p43 in the maternal-fetal tissue interface during normal gestation. Study Design: Villous tissues samples were collected between 5 and 20 weeks’ gestation and at term from uncomplicated pregnancies. Immunohistochemical localization of p43 was performed with CM-H9 monoclonal antibody. Results: During the first trimester p43 was highly expressed in syncytiotrophoblast, Hofbauer cells, and decidual macrophages. From 15 weeks’ gestation onward expression in the syncytiotrophoblast was below the level of detection; however, p43 was demonstrated in villous Hofbauer cells and decidual macrophages throughout gestation. Conclusions: Expression of p43 was demonstrated on both sides of the maternal-fetal tissue interface, with localization dependent on gestational age. This may suggest its immunologic function throughout pregnancy. First-trimester syncytiotrophoblast displayed high p43 levels, which disappeared later on, whereas maternal serum p43 level continued to increase, which suggests an extraplacental source for p43. (Am J Obstet Gynecol 2000;182:670-4.)