Microvesicles (MVs) which include microparticles (MPs) and exosomes are found in blood circulation in normal physiologic conditions and are increased in a variety of diseases. This study evaluated the effects of MVs on human umbilical vein endothelial cells (HUVEC) by morphologic changes, apoptosis, and thrombogenicty, in vitro. Stimulation of monocyte cell line (THP-1) by starvation or by endotoxin and calcium ionophore A23187 resulted in the release of MVs which express exosome marker Tsg 101, negative phospholipids in their leaflets, monocyte markers (CD18, CD14) and active tissue factor (TF). MVs were found to disrupt EC integrity and rapidly induce membrane blebbing. Brief exposure (2-4 hours) to MVs resulted in EC membrane phospholipids "flip-flop" while longer stimulation (20 hours) led to two contradicting outcomes - tube formation as well as apoptosis, as assessed by nuclear fragmentation. Additionally, MVs exposure resulted in increased cell surface thrombogenicity and perturbation of the endothelial haemostatic balance, which were enhanced during longer exposure time. Activity, antigen level and mRNA expression of the coagulation initiator TF were elevated due to (i) adherence of MVs derived TF to the EC membrane, and (ii) an increase in endothelial TF expression. Furthermore, levels of the anticoagulant tissue factor pathway inhibitor (TFPI) and thrombomodulin (TM) were decreased. These findings demonstrate that monocyte MVs increase endothelial thrombogenicity and apoptosis. In addition, they induce tube formation which may indicate their angiogenic effect. These findings may clarify, in part, the role of MVs in EC dysfunction associated with inflammatory diseases and hypercoagulable states.