Cord blood is source of colony-forming progenitors to vascular endothelial cells for potential use in cell therapies. These cells-called blood late outgrowth endothelial cells (OECs)-have undergone endothelial differentiation, but appear to still possess functional properties different from mature endothelial cells. A large-scale comparative proteomics screen of cord blood OECs versus human vein endothelial cells (HUVECs) using two-dimensional gel electrophoresis and mass spectrometry identified specific expression of manganese superoxide dismutase (MnSOD), a key antioxidant enzyme expressed in the mitochondria, in OECs but not in HUVECs. Immunoblotting verified significant MnSOD levels in all OEC isolates tested and maintained throughout passaging. Endothelial function and cell survival/proliferation assays in the presence of high cytotoxic doses of the superoxide generator compound LY83583 showed OECs profoundly better protected against oxidative stress than HUVECs. Such cytoprotective levels of MnSOD cells could give therapeutic cell transplants a survival advantage in necrotic or ischemic conditions.