ABSTRACT: The monoclonal antibodies (mAb) H315, H317, and OKT9 have been used in immunofluorescence to investigate the expression of fetal trophoblast membrane antigens by cells within human term amniochorionic membranes and the marginal area of term placental bed tissue. OKT9 reacted only with trophoblast of placental chorionic villi and did not react with any nonvillous cytotrophoblast population: this mAb is known to identify the cell surface receptor for transferrin. H315 identifies a trophoblast-specific cell-surface antigen and strongly stained both placental villous trophoblast and the cytotrophoblastic layer of amniochorion. This mAb also stained some extravillous cytotrophoblast in the term placental bed, notable interstitial cytotrophoblast within maternal decidua. H317, which identifies placental-type alkaline phosphatase, gave the same distribution pattern as H315.