Measuring membrane permeability is one of the most common methods used to assess cell viability. Lactate dehydrogenase (LDH) is a soluble enzyme located in the cytoplasm. The enzyme is released into the surrounding environment upon cell damage or lysis, processes that occur during both apoptosis and necrosis. LDH activity in the culture medium is therefore used as an indicator of reduced cell membrane integrity and thus serves as a marker in the measurement of cytotoxicity. LDH cytotoxicity assays have routinely been done in two dimensional (2D) culture plates with monolayer cells. There is a growing trend to replace 2D cell culture with 3D cell culture, because the latter better mirrors the environment experienced by normal cells in the body. One such 3D culture format is hanging drop cell culture (developed by 3D Biomatrix) in which cells form spheroids that do not come into contact with plastic. Cell spheroids grown in hanging drops can be maintained and their growth with or without test compounds monitored over a period of up to two weeks. Cayman's Perfecta3D(R) LDH Cytotoxicity Assay provides an easy to use format for studying cytotoxicity in the hanging drop model using a 3D culture plate. The assay is based on a coupled two-step reaction. In the first step, LDH catalyzes the reduction of NAD+ to NADH and the oxidation of lactate to pyruvate. In the second step of the reaction, diaphorase uses the newly-formed NADH to reduce a tetrazolium salt (INT) to its highly-colored formazan derivative, which absorbs strongly at 490-520 nm. The amount of formazan derivative produced is proportional to the amount of LDH in the culture medium.