The monoclonal antibody MEC7.46 recognizes the mouse form of the platelet-endothelial cell adhesion molecule (PECAM)-1 (CD31). PECAM-1 is a member of the immunoglobulin superfamily. This heavily glycosylated protein is found in the entire vascular endothelium of adult mice and functions in mediating cellular adhesion by heterophilic and homophilic mechanisms. PECAM-1 is detected within the lymphopoietic islands in the spleen of newborn (day 12) and in the bone marrow of adult mice. Capillary endothelial cells of adult mice also express PECAM-1.The reactivity of the monoclonal antibody MEC7.46 is restricted to the isoform of the molecule that is selectively expressed by endothelial cells. The antibody precipitates a 130 kDa molecule present on the membrane of endothelial cells of all mouse blood vessels both in normal, inflamed and tumor tissues. The antigen is predominantly present at the lateral borders of endothelial cells as described for human PECAM-1. Staining of MEC7.46 can be seen on capillaries, veins, arteries and liver sinusoids.