It has been postulated that the transplacental passage of maternal iron to the developing fetus requires binding of maternal transferrin to the trophoblast. We have therefore examined the ability of the human placenta to bind transferrin in vitro. Transferrin was demonstrated on trophoblast of human chorionic villi by immunohistologic methods. Moreover, after removal of transferrin bound in vivo by treatment of tissue with chaotropic solution or phosphate-buffered saline, freshly added transferrin was shown to bind in vitro in the same characteristic distribution. These findings suggest that placental iron transport is initiated by uptake of maternal transferrin iron to specific trophoblast binding sites.