The CellTiter-Glo® 3D Cell Viability Assay is designed for determining cell viability in 3D microtissue spheroids. The assay reagent penetrates large spheroids and has increased lytic capacity—allowing more accurate determination of viability compared to other assay methods. Based on the same reliable chemistry as the classic CellTiter-Glo® Assay, this new 3D assay reagent measures ATP as an indicator of viability and generates a luminescent readout that is much more sensitive than colorimetric or fluorescence-based methods. The simple, 30-minute protocol and ready-to-use reagent allows for fast results.