Naturally occurring antiphospholipid (aPL) antibodies against cardiolipin (CL)- and phosphatidylserine (PS)-dependent antigens are associated with placental dysfunction and unsuccessful pregnancy. Murine monoclonal aPL antibodies react with placental trophoblast and may interfere with normal trophoblastic function. In this study, we evaluated the expression of phospholipid-dependent antigens during trophoblast differentiation and measured the effects of monoclonal aPL antibodies on two in vitro aspects of trophoblast differentiation: hormone production and invasion into filters coated with extracellular matrix. Murine monoclonal IgM aPL antibodies that differentiated between PS and CL were used: 3SB9b reacted only with PS (CL-/PS+), D11A4 reacted only with CL (CL+/PS-), and BA3B5C4 reacted with both CL and PS (CL+/PS+). Isolated trophoblasts were cultured for 4 days, and reactivity with monoclonal aPL antibodies was evaluated daily. BA3B5C4 (CL+/PS+) reacted strongly with most trophoblasts that were freshly isolated (Day 0) and through 2 days of culture, after which time the percentage of cells reactive with BA3B5C4 decreased steadily. 3SB9b (CL-/PS+) reactivity increased during incubation; about 8% of cells reacted initially, but after 1 day of incubation 100% reacted, and this percentage remained stable throughout the 4-day incubation. D11A4 (CL+/PS-) reacted only minimally and at the level of the negative control monoclonal antibody (mAb) with 1- and 2-day cultures. Both mAbs that reacted with PS-dependent antigens completely prevented invasion of matrigel-coated filters by isolated trophoblasts. These mAbs also inhibited trophoblastic hCG and human PL production by more than 45%. Thus, as trophoblasts undergo differentiation, they are reactive with mAbs against PS. These antibodies are inhibitory in vitro to trophoblastic hormone production and invasion.