Tolonium chloride is an organic chloride salt having 3-amino-7-(dimethylamino)-2-methylphenothiazin-5-ium (tolonium) as the counterion. It is a blue nuclear counterstain that can be used to demonstrate Nissl substance and is also useful for staining mast cell granules, both in metachromatic and orthochromatic techniques. It has a role as a diagnostic agent, a fluorochrome and a histological dye. It contains a tolonium.