**5.4 Released products from dead cells**

The radioisotope chromium (51Cr) release method was initially developed to detect dead cells and has now gradually been replaced by the LDH release method. Due to increased membrane permeability, the damaged or dying cells release the cytosolic LDH in cell culture medium [71], so the LDH activity is proportional to the number of dead cells. The LDH release assay has been used to measure the activity of cytotoxic lymphocyte (CTL) and natural killer (NK) cells, as well as the cytotoxicity caused by drugs, chemicals, or radiation.
