Toluidine Blue (also known as tolonium chloride or Toluidine blue O) is metachromatic dye used for a variety of histological staining, it selectively stains acidic tissue components (sulfates, carboxylates, and phosphate radicals. Toluidine blue has an affinity for nucleic acids, and therefore binds to nuclear material of tissues with a high DNA and RNA content. It was evaluated the toluidine blue staining in premalignancies, and superficial oral ulceration suggesting malignancy. The study showed 100% sensitivity in the detection of in situ and invasive carcinoma and no false-negative results occurred. The lesions that were diagnosed as dysplasia did not retain stain, and thus gave false-negative results. The reasons could be that the exact mechanisms by which the dye differentially stains malignant or dysplastic tissues remain unknown.