Lactate dehydrogenase is present within the cytoplasm of all mammalian cells. The normal cell membrane is impermeable to LDH and therefore the enzyme is released only if the plasma membrane is damaged. The release of LDH from cells has been used extensively as an accurate marker for cell death and is beginning to replace the more traditional radioactive 5ICr release assay as a test for cell-mediated cytotoxicity. In addition, LDH assays are used routinely in human medicine as diagnostic tests for myocardial injury’ and neoplasia. LDH assays have also been used in preclinical investigations into the biocompatibility of materials for use as surgical implants. Results from our laboratory and from other research groups have confirmed the reliability of the assay as a test for cytotoxicity in vitro.