Toluidine blue solution is used in testing for lignin, a complex organic molecule that bonds to cellulose fibres and strengthens and hardens the cell walls in plants.

Toluidine blue is a basic thiazine metachromatic dye with high affinity for acidic tissue components.

[2] It stains nucleic acids blue and polysaccharides purple and also increases the sharpness of histology slide images.

The strongly acidic macromolecular carbohydrates of mast cells and cartilage are coloured red by the blue dye, a phenomenon called metachromasia.

Alkaline solutions of toluidine blue are commonly used for staining semi-thin (0.5 to 1 μm) sections of resin-embedded tissue.