The pathogenic role of anti-annexin V antibodies remains unclear. Anti-annexin V antibodies are frequently associated with higher incidences of intrauterine fetal loss, preeclampsia, and arterial and venous thrombosis. The present study investigated the in vitro ability of anti-annexin V antibody to bind human trophoblast cells, to affect trophoblast gonadotropin secretion and invasiveness, and to induce placental apoptosis. Cytotrophoblast cells were dispersed in Ringer bicarbonate buffer containing trypsin and DNase I, filtered, and layered over a Percoll gradient in Hanks balanced salt solution. In the case of monoclonal anti-annexin V antibody, the highest binding was found when the cells displayed the greatest amount of syncytium formation. Anti-annexin V antibody, but not its negative control, induced trophoblast apoptosis and significantly reduced trophoblast gonadotropin secretion. These findings suggest that recognition by anti-annexin V antibody of adhered annexin V on trophoblast cell structures might represent a potential pathogenic mechanism by which these antibodies can cause defective placentation.