Ponceau S is the sodium salt of a diazo dye. Ponceau S solution is a suitable reagent for use in electrophoresis studies. The Ponceau S stain comes ready-to-use and is designed for rapid (5 min) staining of protein bands on nitrocellulose or PVDF membranes (Western blots) and also for staining protein on cellulose acetate membranes. Ponceau S staining on nitrocellulose and PVDF is reversible by washing the stained membrane with 0.1M NaOH for 1 minute. The stained and destained membrane can then be immunologically detected (Western blotting).