PTL I is a basic glycoprotein with a molecular weight of about 58,000. This lectin is a dimer of two closely related subunits, and has an isoelectric point above pH 9.5. Binding occurs with carbohydrate structures containing alpha linked galactosamine. Lactose and other beta linked galactosides are poor inhibitors of binding. Tissue staining patterns using this lectin are clearly defined. Stratified squamous epithelium, endothelial cells and underlying basement membranes of blood vessels are intensely stained with PTL I. Pancreatic zymogen granules are also strongly labeled with the lectin.