Methylene blue is used to stain animal cells to make their nuclei more observable. It is a component of staining media such as Wright's stain and Jenner's stain. Methylene blue is used to stain blood films/smears used in cytology and to stain RNA or DNA for viewing under the microscope or on hybridization membranes.

The Löffler's alkaline methylene blue staining method is a simple staining technique that can be used to differentiate bacterial, viral, and fungal infections.

Specifically, it is used to treat methemoglobin levels that are greater than 30% or in which there are symptoms despite oxygen therapy.

a basic thiazine dye C16H18ClN3S·3H2O used especially as a biological stain, an antidote in cyanide poisoning, and an oxidation-reduction indicator.

METHYLENE BLUE AQUEOUS STAINING SOLUTION