Communication between Trypanosoma cruzi (the causative agent of Chagas' disease) and mammalian cells is initiated by contact of parasite surface and cognate host cell molecules. Previously, we have demonstrated that an enzymatically inactive form of T. cruzi trans-sialidase (iTS) behaves as a lectin which binds and triggers contact-dependent activation of NF-kB pathway on endothelial cells. iTS increases expression of adhesion molecules, upregulates parasite invasion of host cells, and rescues endothelial cells from apoptosis by increasing expression of Bcl-2. In this work we aimed at identifying the receptor(s) involved in the interaction between iTS and human bone marrow endothelial cells (HBMEC). Using affinity purification, we demonstrate that iTS specifically binds to a 36 KDa protein from HBMEC lysate. Aminoacid sequencing of tryptic peptides, followed by mass spectrometry analysis (electron spray ionization/ion TRAP), showed an iTS co-receptor which was identified as annexin II with a confidence level approaching 100 %. Flow cytometry studies demonstrated that anti-annexin II antibodies prominently stained the external surface of HBMEC cells and pre-incubation of these cells with iTS, abrogated the binding. Furthermore, antibodies against annexin II decreased iTS-PE binding to HBMEC. To confirm iTS-Annexin II binding, we used enzyme-linked immunosorbant assay (ELISA). Together, these results strongly suggest that iTS interacts with annexin II on the endothelial cell surface. The ability of iTS to recognize annexin II, a molecule abundantly expressed on endothelial cells, may mediate vascular injury observed during T. cruzi infection.