Malachite green oxalate is a synthetic dye used as a counterstain in microscopy, especially in bacteriology and histology. It is a green crystalline solid that is soluble in water and has an intense green color.

In bacteriological staining, such as Ziehl-Neelsen staining for acid-fast bacteria, malachite green oxalate is used to stain the background and non-acid-fast cells green. This creates a contrast that allows easier detection and differentiation of acid-fast bacteria, which are stained bright red by the carbolfuchsin dye.

Malachite green oxalate is also used in histology, for example in the staining of fungi or as a component of trichrome stains. Due to its intense green color, malachite green oxalate gives the stained structures a good contrast and thus facilitates the visualization and differentiation of cell structures and tissue components.