Resazurin is a small N-oxide derivative of dihydroxyphenoxazone. It is a non-ionic dye containing a small and planar chromophore. It is a redox indicator and changes from bluish violet through pink to colorless upon reduction. It is also used as a pH indicator for Ph 3.5 to 6.5 wherein it changed from orange to bluish violet. It is commercially available as resazurin sodium salt, which is also known as 3H-Phenoxazin-3-one, 7-hydroxy-, 10-oxide, sodium salt. It forms the red, fluorescent hydroxyphenoxazone resorufin on reduction and colorless hydroresorufin on further reduction.

For the measurement of metabolic activity and proliferation of living cells. The bioreduction of the dye reduces the amount of the oxidized form (blue) and concomitantly increases the fluorescent intermediate (red).

Resazurin is mostly used as a biological redox indicator. The bioreduction of the dye reduces the amount of the oxidized form (blue) and concomitantly increases the fluorescent intermediate (red). Its applications include:

• the measurement of metabolic activity and the proliferation of living cells
• the detection of bacterial reduction in milk
• in viability assays as a component of culture media for the detection of viable bacteria
• to determine the number of motile sperm in sheep semen
• to assay lactate dehydrogenase
• as an analytical reagent to quantify L-glutamate