Materno-foetal transmission causes one of the most serious forms of infection with the intracellular protozoan parasite Toxoplasma gondii. In the placenta, trophoblast cells constitute the barrier between maternal circulation and foetal tissue. We looked at the factors that determine the extent of cell adhesion to human BeWo trophoblast cells during T. gondii infection. BeWo monolayers stimulated with the supernatant of T. gondii-infected PBMC showed a large increase in THP-1 cell adhesion and upregulation of the intercellular adhesion molecule (ICAM)-1. Neutralization of cytokines by corresponding antibodies demonstrated that anti-IFN-gamma, but not anti-TNF-alpha or anti-IL-1beta, led to a significant reduction of THP-1 adhesion to a BeWo monolayer. Treatment of BeWo cells with single cytokines failed to induce upregulation of adhesion. In contrast, simultaneous treatment with IFN-gamma and either TNF-alpha or IL-1beta mimicked strongly the effect of infected cell supernatant. The results suggest that IFN-gamma plays a pivotal role in the cell adhesion process through upregulation of ICAM-1 and in the process of congenital transmission of T. gondii.