The most common ways to measure cytotoxicity are by measuring ATP content or by using formazan dye or protease biomarkers to assess cell viability. You can also use the lactate dehydrogenase activity (LDH) assay to measure cytotoxicity. Most assays to measure cell cytotoxicity are based on the fact that the cellular membranes of dying cells are damaged or otherwise compromised, allowing components of the cytoplasm to be released out of the cell structure while at the same time allowing fluorescent dyes to enter the cell structure.