Cell death or cytotoxicity is classically evaluated by the quantification of plasma membrane damage. The LDH-Cytotoxicity Assay Kit provides a fast and simple method for quantitating cytotoxicity based on the measurement of activity of lactate dehydrogenase (LDH) released from damaged cells. Unlike many other cytoplasmic enzymes which exist in many cells either in low amount (e.g., alkaline and acid phosphatase) or unstable, LDH is a stable cytoplasmic enzyme present in all cells and rapidly released into the cell culture supernatant upon damage of the plasma membrane. LDH activity can be determined by a coupled enzymatic reaction: LDH oxidizes lactate to pyruvate which then reacts with tetrazolium salt INT to form formazan. The increase in the amount of formazan produced in culture supernatant directly correlates to the increase in the number of lysed cells. The formazan dye is water-soluble and can be detected by spectrophotometer at 500 nm. The LDH-cytotoxicity assay is sensitive, convenient, and precise, and is applicable to a variety of cytotoxicity studies. Assay takes ~0.5–1 hr.