The CD9 molecule is expressed on human extravillous trophoblasts, which invade the endometrium during implantation and placentation. To elucidate the role of CD9 in trophoblastic function, we investigated the expression of CD9 protein and mRNA in BeWo cells, a human trophoblast-like choriocarcinoma cell line, using immunohistochemistry, Western blotting and reverse transcription-polymerase chain reaction (RT-PCR). When BeWo cells were cultured with anti-CD9 monoclonal antibodies (mAb), their invasion through the extracellular matrices was significantly enhanced in a dose-dependent manner. Cell proliferation and human chorionic gonadotrophin production were unaffected. On the other hand, culture in the presence of mAb against integrins alpha3, alpha5 and beta1, which partially block the interaction with the extracellular matrices, inhibited BeWo cell invasion. Anti-CD9 monoclonal antibody had a stimulatory effect on BeWo cell invasion in the presence of anti-integrin alpha3 antibody. In contrast, it had no effect in the presence of mAb against integrins alpha5 and beta1, which were also highly expressed on BeWo cells. These findings suggest that CD9 has a function connected with the invasive properties of BeWo cells, which is partially mediated by integrin alpha5beta1. This may relate to the involvement of CD9 in trophoblastic invasion.