Ulex europaeus agglutinin I, revealed by an immunoperoxidase (PAP) reaction, marks strongly and uniformly, on usual preparations human endothelial cells and makes up one of the best methods to observe vascular network distribution. It defines a general type (UEA I positive, LTA and DBA negative) including most of normal and pathologic endothelial cells. Only three varieties of cells, usually named endothelial ones do not stain: they are liver and marrow sinusoidal cells and lymphatic endothelial cell.