Immunohistochemical staining for human chorionic gonadotropin and factor VIII-related antigen with the avidin-biotin complex immunoperoxidase technique was used as a marker for syncytiotrophoblast and endothelial cells, respectively, in the human placental bed. Material from placental implantation sites at varying stages of gestation (8 weeks to term) was studied. Trophoblastic invasion of the uterine stroma and blood vessels were evaluated. Syncytiotrophoblasts lining placental villi and anchoring villi were positive for human chorionic gonadotropin at all stages of gestation studied. Endothelial cells lining maternal uterine blood vessels were positive for factor VIII-related antigen. At early stages of intrauterine placentation (8 and 11 weeks) trophoblastic invasion of uterine blood vessels and trophoblastic incorporation in the walls of dilated vessels were present. An unexpected finding, however, was the large number of giant cells in the superficial placental bed which had morphology suggestive of syncytiotrophoblast but which were negative for human chorionic gonadotropin. In addition, many enlarged, rather pleomorphic cells lining superficial blood vessels were found to be positive for factor VIII-related antigen, which identified them as endothelial cells and not migrating trophoblastic elements. This study demonstrates that human chorionic gonadotropin and factor VIII-related antigen immunoperoxidase staining is a helpful adjunct in evaluating human placentation and suggests extension of the technique with use of other antibodies to evaluate components of the placental bed.