We have tested the hypothesis that elevated concentrations of TNF alpha could impair trophoblast invasion. Using first-trimester placental explant cultures, we have demonstrated that the cytokine inhibits in vitro migration of extravillous trophoblasts (EVT) on collagen I, and invasion through Matrigel. To elucidate the underlying mechanism, proliferation and differentiation of EVT in vitro were analyzed by immunohistochemistry of serial sections, Western blotting, zymography, ELISA, and RT-PCR from RNA pools of mechanically separated cell populations. At 24 h of cultivation in the presence or absence of TNF alpha, anchorage and proliferation of trophoblasts had occurred to generate cell columns containing viable, post-mitotic, differentiated EVT [positive for integrins alpha 1 and alpha 5, matrix metalloproteinase (MMP)-2, and human leukocyte antigen-G1; negative for proliferating cellular nuclear antigen, cytokeratin 18 neoepitope, and in 5-Bromo-2-deoxy-uridine labeling]. At 72 h, control cells had broken away from the column to migrate through the extracellular matrix; whereas, in contrast, TNF alpha-treated EVT remained as contiguous cell columns, despite increased MMP-9 expression. Thus, in vitro MMP9 activity appears not to be essential for trophoblast migration. Expression of plasminogen activator inhibitor (PAI)-1 was elevated in TNF alpha-treated EVT, and adding antibodies that inhibit PAI-1 activity restored migration, whereas tissue-inhibitor-of-metalloproteinases-1-blocking antibodies were ineffective. Induction of PAI-1 by TNF alpha could be related to restricted trophoblast invasion in preeclampsia.