The FungaLight™ Yeast CFDA, AM / Propidium Iodide Vitality Kit combines a cell permeable esterase substrate with a membrane integrity indicator to evaluate the vitality of yeast cells by flow cytometry or microscopy. The acetoxymethyl ester (AM) of the esterase substrate 5-carboxyfluorescein diacetate (CFDA) allows this reagent to permeant cell membranes Once inside the cell, the lipophilic blocking and diacetate groups are cleaved by nonspecific esterases, resulting in a fluorescent, charged form that leaks out of cells very slowly. In contrast, the membrane integrity indicator, propidium iodide penetrates yeast with damaged membranes. With an appropriate mixture of the CFDA, AM and propidium iodide stains, esterase-active yeast with intact cell membranes stain fluorescent green, whereas yeast with damaged membranes stain fluorescent red.