Eosin 1 % in methanol is an alcoholic solution of the synthetic acid dye eosin, which is used in histology and cytology for staining cell structures. At a concentration of 1%, this solution is specifically designed to stain acidophilic (basic or eosinophilic) structures, which it stains red. This includes cell plasma proteins, mitochondria, smooth endoplasmic reticulum, collagen and keratin.

Eosin is often used in combination with other stains, such as hematoxylin in hematoxylin-eosin (HE) staining. This combination enables differentiated visualization of various cell structures and tissue components. In this process, hematoxylin stains the cell nuclei blue and eosin stains the remaining cell structures red.

In addition, the eosin solution finds application in microbiology, where it is used to stain bacteria and fungi.