Eosin 10%, methanolic is a solution containing the synthetic acidic dye eosin in methanol. Eosin is known for its ability to stain acidophilic (basic or eosinophilic) structures in tissue sections, giving them a characteristic red color.

In histology, eosin is used as a component of hematoxylin-eosin staining, one of the most widely used staining methods in histology. Its special affinity is for cell plasma proteins, mitochondria, smooth endoplasmic reticulum, collagen, and keratin, which it stains in varying shades of red.
In addition, eosin is particularly useful in eosinophil staining. Here, it specifically stains eosinophil granulocytes, cells that play an essential role in inflammatory and allergic reactions.

The intensity of eosin staining can be controlled by the duration of exposure of the tissue section to the dye and the concentration of the eosin solution. Longer exposure and higher concentration usually result in more intense staining.

Outside of histology, the Eosin 10%, methanolic solution also finds application in microbiology, where it is used to stain microorganisms.