Masson – Fontana stain is based on the properties of melanins. Melanins are a group of brown-black precipitates in the exact chemical structure is not known.

Melanins bind to proteins and melanin-protein complexes are localized in the cytoplasm of cells in melanin granules. Melanin has the ability to reduce ammonia silver nitrate solutions to metallic silver without the use of external reducing agent. Masson - Fontana stain is far from being specific for melanin, because it also makes it possible to visualize other substances which have the same properties of reduction, as the argyrophilic granules and the lipofuschines.

Masson - Fontana stain can be used for the differentiation of brown pigments (lipofuschine, hemosiderin, melanin), for the deposition of pigment due to treatments like Minocycline type II or for the differential diagnosis of hypomelanosis.

Here are some examples of coloration obtained with the method of Masson - Fontana: