A monoclonal antibody, or fragments thereof, having binding specificity for both endoglin expressed on human vascular endothelial cells and on murine endothelial cells. Antiangiogenic therapy in mammals can be effected by administering to the mammalian host a therapeutically effective amount of an anti-endoglin monoclonal antibody, or fragment thereof, which is conjugated to at least one angiogenesis inhibitor or antitumor agent. The composition is useful for treating tumor and angiogenesis-associated diseases.