Eriochrome Cyanine R belongs to anionic sulfonphthalein mordant dyes. It can be used independently as pH indicator or as red anionic dye; however, it creates intensely stained complexes with transition metal ions (such as iron ions), and because of that this dye is most commonly used in histology as hematoxylin substitute. Eriochrome represents economic and ecologically acceptable synthetic replacement for hematoxylin, and its working solutions show superior stability compared to hematoxylin working solutions. A mix of Eriochrome Cyanine R and ferrous mordant (Fe) is used for staining. Eriochrome and ferrous ions create 4 different complexes, two of which are red, and two are blue. If these dyes’ solutions with ferrous salts are used under suitable conditions, selective nuclear staining may be achieved, similar to hematoxylin staining results. This method is compatible with the usual eosin counterstaining as a substitution for HE staining, but it is also used in special staining kits, such as: A.F.O.G., Gomori Trichrome, Masson-Goldner Trichrome, Masson Trichrome, Picro Sirius Red, Safranin O and Mucicarmine.