INTRODUCTION: Because Ponceau S is relatively insensitive (~1 μg of protein), only the most abundant proteins will be visible. However, it is a reversible stain that can be removed completely with H2O prior to processing the blots. After staining, a soft lead pencil can be used to record the presence of visible proteins and molecular-weight markers, which will help when aligning the proteins detected on the membrane by western analysis with those in a total protein-stained gel or membrane. Ponceau S is compatible with both nitrocellulose and PVDF membranes.