Acid blue 93 (C.I. 42780)

Methyl blue is a chemical compound with the molecular formula C₃₇H₂₇N₃Na₂O₉S₃. It is used as a stain in histology, and stains collagen blue in tissue sections. It can be used in some differential staining techniques such as Mallory's connective tissue stain and Gömöri trichrome stain, and can be used to mediate electron transfer in microbial fuel cells. Fungal cell walls are also stained by methyl blue.

Methyl blue is also available in mixture with water blue, under name Aniline Blue WS, Aniline blue, China blue, or Soluble blue; and in a solution of phenol, glycerol, and lactic acid under the name Lactophenol cotton blue (LCB), which is used for microscopic visualization of fungi