Objective— Increased levels of circulating tissue factor (TF) in the form of microparticles increase the risk of thrombosis. However, any direct influence of microparticle-associated TF on vascular endothelial cell proliferation is not known. In this study, the influence of recombinant and microparticle-associated TF on endothelial cell proliferation and mitogen-activated protein kinase signaling mechanisms was examined. Methods and Results— Incubation of human coronary artery endothelial cells with lipidated recombinant full-length TF, or TF-containing microparticles (50 to 200 pmol/L TF), increased the rate of cell proliferation and induced phosphorylation of extracellular signal regulated kinase 1 in a TF-dependent manner. Inhibition of extracellular signal regulated kinase 1/2 using PD98059 or extracellular signal regulated kinase 1/2 antisense oligonucleotides or inhibition of c-Jun N-terminal kinase reduced recombinant TF-mediated cell proliferation. PD98059 also reduced cell proliferation in response to TF-containing microparticles. Inclusion of FVIIa (5 nmol/L) and FXa (10 nmol/L) or preincubation of cells with an inhibitory anti-FVIIa antibody had no additional influence on TF-mediated cell proliferation. However, preincubation of exogenous TF with a β1-integrin peptide (amino acids 579 to 799) reduced TF-mediated proliferation. Conclusion— High concentrations of recombinant or microparticle-associated TF stimulate endothelial cell proliferation through activation of the extracellular signal regulated kinase 1/2 pathway, mediated through a novel mechanism requiring the interaction of exogenous TF with cell surface β1-integrin and independent of FVIIa.