Propidium iodide (PI) is a popular red-fluorescent nuclear and chromosome counterstain. Since propidium iodide is not permeant to live cells, it is also commonly used to detect dead cells in a population.

PI binds to DNA by intercalating between the bases with little or no sequence preference. In aqueous solution, the dye has excitation/emission maxima of 493 / 636 nm. Once the dye is bound, its fluorescence is enhanced 20- to 30-fold, the fluorescence excitation maximum is shifted ~30–40 nm to the red and the fluorescence emission maximum is shifted ~15 nm to the blue, resulting in an excitation maximum at 535 nm and fluorescence emission maximum at 617 nm.

PI is widely used in fluorescence microscopy, confocal laser scanning microscopy, flow cytometry, and fluorometry.

Propidium iodide dashboard

Initial brightness



Prodium iodide is impermeant to live cell membranes and exhibits enhanced fluorescence upon binding to either DNA or RNA.


Photostability in buffer


Photostability in antifade


  488 Texas Red® 535 617  
Laser line

Common filter set

Excitation max

Emission max

Propidium iodide products

Propidium iodide has proven valuable in a broad range of research applications with multiple analytical platforms:

Propidium Iodide

A human pancreatic islet stained with Newport Green™ DCF diacetate and propidium iodide.

Dead cells are identified by red-fluorescent propidium iodide staining. Image contributed by Brigitte Vandewalle, University of Lille, and reproduced with permission from J Histochem Cytochem 49, 519 (2001).

High-performance nucleic acid stains

Nuclear counterstains provide highly selective nuclear staining with little or no cytoplasmic labeling and a choice of colors for multiplexing with other labels.  

Some nuclear stains can also be used in cell cycle analyses, and others serve as dead cell stains, using high-content imaging and flow cytometry.


FluoCells prepared slide #6 showing a fixed, permeabilized, and labeled muntjac skin fibroblast.

Mitochondria were labeled with mouse anti–OxPhos Complex V inhibitor protein antibody and visualized using orange-fluorescent Alexa Fluor® 555 goat anti–mouse IgG antibody. F-actin was labeled with green-fluorescent Alexa Fluor® 488 phalloidin, and the nucleus was stained with TO-PRO®-3 iodide (pseudocolored magenta).



Molecular Probes® Handbook

Molecular Probes HandbookThe significantly revised 11th Edition is the most complete fluorescence labeling and detection reference available. The Molecular Probes® Handbook can help you achieve the best experimental results for your fluorescence labeling and detection experiments. View the handbook online or order your own copy.


Antifade reagents

Antifade reagentsAntifade reagents suppress photobleaching and preserve the signals of your fluorescently labeled target molecules. They offer excellent compatibility with a multitude of dyes across the spectrum, making them especially valuable tools for multicolor applications. For added convenience, some formulations include the nuclear counterstain DAPI, eliminating the need for a separate counterstaining step.

Cell stain tool

Cell stain toolStain your own cell by using our simulation tool to develop reproducible results with many of our signature fluorescent dyes. Develop your multiplexing strategy for antibody probes, reactive dyes, and counterstains to create your perfectly labeled fluorescent cell.


Fluorescence SpectraViewer

Fluorescence SpectraviewerPlot and compare spectra and check the spectral compatibility for common fluorophores including all the Molecular Probes® fluorescent probes. The Fluorescence SpectraViewer tool is formatted for both microscope imaging and flow cytometry applications.