Endothelial cells are an attractive vehicle for gene therapy because they may be used in an autologous fashion and may allow for direct exposure of the gene product into the intravascular space. To explore this future potential, a reproducible system was developed for the culture of murine blood outgrowth endothelial cells. These cells demonstrated acetylated low-density lipoprotein (LDL) incorporation, matrigel tube formation, and specific endothelial staining characteristics, namely P1H12, VeCAD, vascular cell adhesion molecule (VCAM), vWF, platelet endothelial cell adhesion molecule (PECAM-1), and vascular endothelial growth factor receptor-2 (VEGFR2). They were also negative for smooth muscle actin and monocytic markers CD11b, CD14, and CD16. Moreover, these cells were amendable to gene transfer with red fluorescent and green fluorescent expression vectors as well as human Factor VIII (hFVIII) while maintaining endothelial characteristics. Both source- and gene-introduced cells also manifested excellent proliferative potential. Furthermore, murine blood outgrowth endothelial cells (BOECs) demonstrated persistent in vivo seeding in the liver, lung, spleen, and bone morrow of recipient mice.