Propidium iodide is a cell-permeant stain that binds to nucleic acids. Nonviable cells and cells with damaged membranes are the only ones that can take up propidium iodide, since intact membranes cannot penetrate it. Once bound to nucleic acids, its fluorescence is multiplied by 20-30, causing the cell to fluoresce red.

The LUNA™ Automated Fluorescent Cell Counter can use propidium iodide stain (F23003) with acridine orange stain (F23002)to assess cell viability. Viable nucleated cells with fluoresce green and non-viable cells will fluoresce red. Due to Förster resonance energy Transfer (FRET), the signal from propidium iodide absorbs the signal from acridine orange in non-viable cells, ensuring that double-positive results do not occur.