Bevacizumab (Avastin, Genentech) is a humanized monoclonal antibody targeting vascular endothelial growth factor (VEGF), a critical angiogenic factor involved in both physiological and pathological conditions. It has been recently approved by the US FDA as a first-line therapy for widespread metastatic colorectal cancer. This report is a detailed biological characterization of bevacizumab in a variety of in vitro models. It is shown that bevacizumab potently neutralizes VEGF and blocks its signal transduction through both the VEGFR-1 and VEGFR-2 receptors, as demonstrated by the inhibition of VEGF-induced cell proliferation, survival, permeability, nitric oxide production, as well as migration and tissue factor production. Although bevacizumab retains the ability to bind to human Fcgamma receptors and complement protein C1q, it does not demonstrate cell or complement-mediated cytotoxicity in either VEGF producing or targeting cells. Thus the mechanism of anti-tumor activity of bevacizumab is most likely due to its anti-angiogenesis effect through binding and neutralization of secreted VEGF.