OBJECTIVE To investigate the binding of antibodies against negatively charged phospholipids (antiphospholipid antibodies) to human placenta, we tested the reactivity of three mouse monoclonal antiphospholipid antibodies against first-trimester human placenta. METHODS Formalin-fixed and frozen sections of first-trimester placentas were stained by immunoperoxidase with three mouse monoclonal antibodies. Each monoclonal antibody reacted differently with cardiolipin and phosphatidylserine, 3SB9b reacted with phosphatidylserine, D11A4 reacted with cardiolipin, and BA3B5C4 reacted with both. RESULTS 3SB9b reacted strongly with the syncytiotrophoblastic layer of both formalin-fixed and frozen placental tissue. Sporadic reactivity was observed against the cytotrophoblastic layer. BA3B5C4 reacted strongly and specifically with cytotrophoblastic cells. D11A4 reacted minimally or, more commonly, not at all. CONCLUSIONS The trophoblastic layer directly in contact with the maternal circulation is most reactive with antiphospholipid antibodies that react with phosphatidylserine rather than cardiolipin, suggesting that the trophoblasts may potentially be directly damaged by antiphospholipid antibodies through mechanisms unrelated to thrombosis. In addition, the differential reactivity of 3SB9b and BA3B5C4 suggests that the antigenic conformation involving phosphatidylserine on the cytotrophoblast is altered concurrent with fusion into the syncytium.