The MTT Cell Viability Assay Kit provides a simple method for determining live cell numbers using a standard colorimetric plate readers. Determination of live cell numbers is often used to assess the rate of cell proliferation and cytotoxicity caused by drugs and cytotoxic agents. Among all non-radioactive viability assays, MTT assay developed by Mossman is one of the most versatile and popular assays. MTT is a tetrazolium salt that is turned into a purple formazan product after reduction by mitochondrial enzymes that are only present in metabolically active live cells, not in dead cells. The amount of formazan product generated is proportional to the number of living cells in the sample. At the end of the assay, the cells containing the formazan product are solubilized and then photometrically quantified at 570 nm.     Also see the Biotium's XTT Cell Viability Assay Kit, which does not require solubilization before absorbance measurement.