Methylene Blue 1%, aqueous with TWEEN 80, is primarily used in histology and in vitro diagnostics. The dye methylene blue, chemically designated as C16H18ClN3S, has the ability to stain certain biological structures, especially those containing acidic components. This is due to the basic nature of the dye, which binds to acidic tissue components. This involves an interaction between the positively charged molecules of the methylene blue and the negatively charged components of the tissue, mostly polysaccharides and nucleic acids. TWEEN 80, an emulsifier and surfactant, provides a more uniform distribution of the dye, while sodium azide acts as a preservative. The result of using this solution is improved visualization of specific cell structures and facilitates accurate diagnosis in medical and scientific applications.