Neutral red

Neutral red (toluylene red, Basic Red 5, or C.I. 50040) is a eurhodin dye used for staining in histology. It stains lysosomes red.[1] It is used as a general stain in histology, as a counterstain in combination with other dyes, and for many staining methods. Together with Janus Green B, it is used to stain embryonal tissues and supravital staining of blood. Can be used for staining Golgi apparatus in cells and Nissl granules in neurons.

Neutral red
Solid Neutral Red
Neutral Red Aqueous Solution
IUPAC name
3-Amino-7-dimethylamino-2-methylphenazine hydrochloride
Other names
toluylene red
3D model (JSmol)
ECHA InfoCard 100.008.215
EC Number
  • 209-035-8
Molar mass 288.78 g/mol
Melting point 290 °C (554 °F; 563 K)
GHS pictograms
GHS Signal word Danger
H301, H315, H319, H335, H341
P201, P202, P261, P264, P270, P271, P280, P281, P301+310, P302+352, P304+340, P305+351+338, P308+313, P312, P321, P330, P332+313, P337+313, P362, P403+233, P405, P501
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references
Neutral red (pH indicator)
below pH 6.8 above pH 8.0
6.8 8.0

In microbiology, it is used in the MacConkey agar to differentiate bacteria for lactose fermentation.

Neutral red can be used as a vital stain.[2] The Neutral Red Citotoxicity Assay was first developed by Dr. Ellen Borenfreund in 1984. In the Neutral Red Assay live cells incorporate neutral red into their lysosomes. As cells begin to die, their ability to incorporate neutral red diminishes. Thus, loss of neutral red uptake corresponds to loss of cell viability.[3] The neutral red is also used to stain cell cultures for plate titration of viruses.

Neutral red is added to some growth media for bacterial and cell cultures. It usually is available as a chloride salt.

Neutral red acts as a pH indicator, changing from red to yellow between pH 6.8 and 8.0.


  1. Winckler, J. Vital staining of lysosomes and other cell organelles of the rat with neutral Red. Prog. Histochem. Cytochem. 6, 1–89 (1974).
  2. Repetto G, del Peso A, Zurita JL (2008). "Neutral red uptake assay for the estimation of cell viability/cytotoxicity". Nature Protocols. 3 (7): 1125–1131. doi:10.1038/nprot.2008.75. PMID 18600217.CS1 maint: multiple names: authors list (link)
  3. Borenfreund E., Puerner J.A. (1984) A simple quantitative procedure using monolayer cultures for cytotoxicity assays (HTD/NR90). Journal of Tissue Culture Methods 9(1):7-9.

Other references

  • Borenfreund L and JA Puerner. (1985) Toxicity determined in vitro by morphological alterations and neutral red absorption. Journal of Tissue culture methods 9(1):7-9. Toxicology Letters 24(2-3):119-124.
  • Borenfreund E., Babich H., Martín-Alguacil N. (1988) Comparisons of 2 in vitro cytotoxicity assays the Neutral Red (NR) and Tetrazolium MTT Tests. Toxicology In Vitro 2(1):1-6.
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