Determination of live cell numbers is often used to assess the rate of cell proliferation and cell cytotoxicity caused by drugs and cytotoxic agents. The PromoKine Cell Viability Kit IV (MTT) provides a simple method for determining live cell numbers using a standard colorimetric plate reader. Among all non-radioactive viability assays, the MTT assay developed by Mossman is one of the most versatile and popular assays. MTT is a tetrazolium salt that is converted into a purple formazan product after reduction by mitochondrial enzymes that are only present in metabolically active live cells, but not in dead cells. The amount of formazan product generated is proportional to the number of living cells in the sample. The formazan product can be solubilized and then photometrically quantified at 570 nm.