Neutral red is a synthetic red dye used in histology, cytology and microbiology. Chemically, Neutral Red belongs to the class of azo dyes and has the C.I. number 50040. In aqueous solution, the dye exhibits a yellow-red color.

In histology and cytology, Neutral Red is used to stain cell structures such as nuclei, cell membranes, and cell organelles. The dye binds to acidic components of cell structures and allows differentiation of tissue types and cell types.

In microbiology, Neutral Red serves as a pH indicator in culture media and agar plates. At a pH below 6.8, Neutral Red appears red, while at a pH above 6.8 it is yellow. This enables different microorganisms to be distinguished on the basis of their influence on the surrounding medium and their acid or base production behavior.

Neutral red is also used in combination with other dyes to visualize several cell structures and tissue components simultaneously, for example in the staining of bacteria and fungi or in the vital staining of cells.