Angiogenesis is a process associated with the migration and proliferation of endothelial cells (EC) to form new blood vessels. The plasminogen activator–plasmin system plays a major role in the extracellular matrix remodeling process necessary for angiogenesis. Urokinase/tissue-type plasminogen activators (uPA/tPA) convert plasminogen into the active enzyme plasmin, which in turn activates matrix metalloproteinases and degrades the extracellular matrix releasing growth factors and proangiogenic molecules such as the vascular endothelial growth factor (VEGF-A). The plasminogen activator inhibitor-1 (PAI-1) is the main inhibitor of uPA and tPA, thereby an inhibitor of pericellular proteolysis and intravascular fibrinolysis, respectively. PAI-1, which is expressed by EC during angiogenesis, is elevated in several cancers and is found to promote angiogenesis by regulating plasmin-mediated proteolysis and by promoting cellular migration through its interaction with vitronectin. Thus, PAI-1 remains one of the major targets to be inhibited in several pathologies in general and in cancer in particular. We have previously described a physiological inhibitor of PAI-1 which is a 14 KDa fragment of human growth hormone (hGH) proteolytically cleaved from its 22kD full-length precursor by plasmin and thrombin. To test the antiangiogenic and antimetastatic effects of this fragment, MDA-MB231 breast cancer cells were transfected with expression vectors carrying 14 KDa hGH and were used in various in vitro and in vivo experiments. Human Brain Microvascular Endothelial (HBME) cells were also treated with 14 KDa hGH derived peptides which are shorter, more stable, and less expensive alternatives. The effects of 14 KDa hGH derived peptides on HBME cell's proliferation, migration, apoptosis, and angiogenesis were also tested in vitro. Full-length 14 KDa hGH significantly reduced the proliferation of MDA-MB231 breast cancer cells in vitro but failed in reducing primary tumor growth and metastasis in vivo. On the other han