Toluidine blue (Toluidine Blue O) is a commonly used synthetic dyes, belongs to the quinone imine dyes, these dyes mainly contain amine groups and quinone-type benzene ring two chromophores, so that the chromogen color, toluidine blue cations in the staining effect of the tissue cells of the acid and the combination of the cations and be stained; Toluidine blue also contains two auxiliary chromophores, which can promote the dye to produce Toluidine blue also contains two chromophores, which can cause the dye to ionize into salts, help the chromophores to produce staining force on the tissue, so that the tissue cells on the section coloring, can be stained cell nuclei to make it blue.

In addition, mast cell cytoplasm contains heparin and histamine and other heterochromatic substances, encounter toluidine blue can be heterochromatic purplish red, often used for mast cell staining in the clinic, cartilage staining solution (toluidine blue method) is strongly alkaline, more conducive to cartilage tissue coloring.