Ponceau-S Stain also referred to as negative stain is a ready–to–use, reversible staining solution formulated for fast staining of protein bands on nitrocellulose or PVDF membranes during Western blotting procedure as well as cellulose acetate membranes. It stains protein bands red with a clear background and can be washed off under running water. It is composed of 0.5% (w/v) Ponceau S in 1% (v/v) glacial acetic acid.