Cell death or cytotoxicity is classically evaluated by the quantification of plasma membrane damage. Lactate dehydragenase (LDH) is a stable enzyme, present in all cell types, and rapidly released into the cell culture medium upon damage of the plasma membrane. LDH, therefore, is the most widely used marker in cytotoxicity study. The LDH Cytotoxicity Assay Kit II utilizes the advanced WST reagent for a fast and more sensitive detection of LDH released from damaged cells. The assay utilizing an enzymatic coupling reaction: LDH oxidizes lactate to generate NADH, which then reacts with WST to generate yellow color. The intensity of the generated color correlates directly with the cell number lysed. Since WST is brighter, less amount of culture medium is required for the assay, and thus the background from serum and culture medium is significantly reduced. Using the assay, cells can be cultured in regular 10% serum containing medium, no reducing serum or special medium is required for the assay. In addition, since the WST is more stable, the reaction can be read multiple times, and can also be stopped at any time point during the reaction. LDH activity can be easily quantified by spectrophotometer or plate reader at OD450 nm. The kit provides all necessary reagents including LDH positive control. Assay takes less than 1 hour.