2,3,5-Triphenyltetrazolium chloride is a colorless water-soluble dye. In the mitochondria of living cells, it is reduced to a deep red, water-insoluble compound (formazan). 2,3,5-Triphenyltetrazolium chloride helps to distinguish between viable and infarcted brain tissue after stroke.

2,3,5-Triphenyltetrazolium chloride has been used to stain heart tissue to measure the extent of acute lesion. It has also been used to stain brain tissue to check the size of infarct area.