A vital dye used in biological research. One use is in determining the viability of pollen grains; grains which are alive take up the fluorescein diacetate and hydrolyze it intracellularly; the released fluorescein will fluoresce, and can be detected with a microscope using filters that allow only the longer-wavelength light to pass.

My guess is that the same principle can be used with other types of cells to determin their viability.