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W e investigated the mechanisms of alphafetoprotein (AFP) transfer across the human placenta by correlating measurements of AFP transfer with cytochemical localization of AFP. Placental cotyledons were dually perfused in vitro with either the fetal or maternal perfusate containing umbilical cord plasma as a source of AFP. Steady state AFP clearance, corrected for release of endogenous AFP, was 0.973 +/- 0.292 microliter/min per gram in the fetal to maternal direction (n = 10), significantly higher (P < 0.02) than that in the maternal to fetal direction (n = 5; 0.022 +/- 0.013 microliter/min per gram). Clearance of a similarly sized protein, horseradish peroxidase was also asymmetric but clearance of the small tracer creatinine was not. Using a monoclonal antibody, we localized AFP to fibrinoid deposits in regions of villi with discontinuities of the syncytiotrophoblast, to cytotrophoblast cells in these deposits, to syncytiotrophoblast on some villi, and to trophoblast cells in the decidua. We conclude that AFP transfer in the placenta is asymmetric and that there are two available pathways for AFP transfer: (a) from the fetal circulation into the villous core and across fibrinoid deposits at discontinuities in the villous syncytiotrophoblast to enter the maternal circulation; and (b) AFP present in the decidua could enter vessels that traverse the basal plate.