Assay measures cell viability. It is a two-color fluorescence assay that simultaneously determines Live cell number and Dead cell number. This protocol is designed for use with the GEMINI XS Microplate Spectrofluorometer, a multi-well plate scanner with dual excitation/emission capabilities, but the assay is also adaptable for flow cytometry and fluorescence microscopy. Includes: Cell Culture; Preparation for the Assay; Live/Dead Assay; Reading the Plate; Data Analysis; Alternative protocol.