A humanized IgG1 monoclonal antibody directed against human endosialin/TEM1 (tumor endothelial marker;CD248) with potential anti-angiogenic and antineoplastic activities. Anti-endosialin/TEM1 monoclonal antibody MORAb-004 binds to and inhibits the activity of cell surface protein endosialin/TEM1, which may result in the inhibition of angiogenesis, tumor cell proliferation and metastasis. Endosialin/TEM1 plays a key role in angiogenesis and may be overexpressed on tumor stromal cells and endothelial cells.