MTT assay kits for cell viability and cytotoxicity provide a practical, sensitive, quantitative and reliable test for determining the number of viable cells in a given culture. This homogeneous colorimetric assay is based on the conversion of a tetrazolium salt MTT to formazan. MTT is a pale yellow substrate and formazan a violet dye.

This cell reduction reaction involves the NADH / NADPH pyridine nucleotide cofactors and is only catalyzed by living cells. The formazan product has low aqueous solubility and is present as purple crystals. Dissolving the resulting formazan with a solubilization buffer allows appropriate quantification of product formation. The color intensity of the product, measured at 550 - 620 nm, is directly proportional to the number of living cells in the culture. This type of test is easily automated to perform large-scale assays.