The monoclonal antibody RECA-1 reacts with Rat Endothelial Cell Antigen (RECA), a cell surface antigen (MCA970R) on rat endothelial cells. Endothelial cells (EC) line the interior of all blood vessels and are the key players in the angiogenesis cascade. EC are the first cells and barrier that vehicles or medicines encounter after systemic delivery. Furthermore, they have a signaling function to the cells of the immune system to indicate the status of the surrounding tissue. RECA-1is at least reactive with the rat MHC-haplotype; Lewis (TR-11), BN (RT-1n) and OA (RT-1u). RECA-1 antibody has been successfully applied in staining of viable endothelial cells in vitro, and of vascular endothelium in vivo. No reactivity of the RECA-1 monoclonal antibody was seen with other cell types e.g. fibroblasts, leukocytes and non endothelial stromal cells nor with other various tested species other than rat e.g. mouse, rabbit, sheep, goat and human. RECA-1 is a promising antibody for rat endothelial cell studies, and in particular for further defining nature and function of endothelial cell-specific antigens