TO-PRO-3 stain

TO-PRO-3 stain is an excellent far red-fluorescent nuclear and chromosome counterstain that is impermeant to live cells but penetrates compromised membranes characteristic of dead cells, making it a useful indicator of dead cells within a population. The carbocyanine-based dye exhibits far-red fluorescence with excitation at 642 nm and emission at 661 nm. The long-wavelength fluorescence of TO-PRO-3 stain is well separated from that of commonly used fluorophores imaged in the DAPI and FITC channels.

Long-wavelength light–absorbing dyes such as TO-PRO-3 stain have the advantage that their fluorescence is usually not obscured by tissue autofluorescence. TO-PRO-3 stain has very strong binding affinity for dsDNA, with dissociation constants in the micromolar range. It gives strong and selective staining of the nucleus in cultured cells and in paraffin sections. 

TO-PRO-3 stain is among the most sensitive probes for nucleic acid detection, and in addition to cell staining it can be used for staining nucleic acids on solid supports and pre-staining samples for gel or capillary electrophoresis.

TO-PRO-3 stain dashboard

Initial brightness

TO-PRO-3 stain is a nucleic acid stain with far-red fluorescence that is a very sensitive nuclear counterstain and dead cell indicator. 

Photostability in buffer


Photostability in antifade


TO-PRO-3 stain products

We offer TO-PRO-3 stain as an iodine salt in DMSO. TO-PRO-3 stain is also included in the SelectFX Nuclear Labeling Kit, which provides four spectrally distinct fluorescent dyes for staining nuclei in fixed-cell preparations.

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Fluorescence immunostaining of a fixed co-culture of murine neurons and dendritic cells. Fluorescence immunostaining of a fixed co-culture of murine neurons and dendritic cells. Neurons were labeled with an antibody directed against neurofilament triplet H (NFH) protein and visualized using red-fluorescent tetramethylrhodamine goat anti–mouse IgG. Dendritic cells were labeled with a biotin conjugate of a primary antibody directed against the integrin CD11c and visualized using green-fluorescent Alexa Fluor 488 streptavidin (S11223, S32354). Nuclei were stained with blue-fluorescent TO-PRO-3. Image submitted by Gauthier Dorban, Institute of Human Histology, University of Liège, Belgium.

High-performance nucleic acid stains

Unlike our range of nuclear counterstains, SYTO dyes do not act exclusively as nuclear stains in live cells and should not be equated with DNA-selective compounds (such as DAPI) which stain nuclei in live animal cells. Eukaryotic cells incubated with SYTO dyes generally show cytoplasmic or mitochondrial staining, as well as nuclear staining.

Nuclear counterstains provide highly selective nuclear staining with little or no cytoplasmic labeling and a choice of colors for multiplexing with other labels. Additionally, some of the stains can also be used in cell cycle analyses using high-content imaging and flow cytometry.

TO-PRO-3 iodide stained cell

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 IgGantibody. F-actin was labeled with green-fluorescent Alexa Fluor 488 phalloidin, and the nucleus was stained with TO-PRO-3 iodide (pseudocolored magenta).


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