One of the earliest and most common methods for measuring cell viability is the trypan blue staining assay. Trypan blue is an azo dye that is cell membrane impermeable and therefore only enters cells with compromised membranes. Upon entry into the cell, trypan blue binds to intracellular proteins thereby rendering the cells a blue color. The trypan blue staining assay allows for a direct identification and enumeration of live (unstained) and dead (blue) cells in a given population.
