AbstractTo find out whether or not mammalian cells have a surface coat, over 50 cell types were examined in rat tissues stained with the periodic acid‐Schiff technique for glycoproteins or the colloidal iron technique for acidic carbohydrates.With both techniques, nearly all cells investigated are outlined by a thin, but definite band of stained material, indicating the existence of a surface layer. The surface layer is uniform in leucocytes, fibrocytes and other cells of mesenchymal origin. This is true in neurons too, although associated structures may also be stained. In simple epithelia, the layer appears thicker at apical than at lateral and basal surfaces. (At the basal surface, the layer separates the cell from the basement membrane, which is itself colloidal iron negative and therefore is not part of the cell coat.) Finally, the layer is usually interrupted at the tight junction of terminal bars (where the cell interspace disappears as the plasma membranes of adjacent cells fuse). This finding confirms that the layer is not part of the plasma membrane itself but is a surface ‘cell coat.’In agreement with biochemical data, the staining properties indicate the presence of glycoprotein(s) and acidic residues in the coat of rat