Ponceau de Xylidine 1 % (Masson B) is a staining solution used as an ingredient in Masson trichrome staining. Ponceau de Xylidine is a synthetic acid dye present in a 1% aqueous solution. In Masson trichrome staining, it is used to stain cytoplasmic structures, muscle tissue, and erythrocytes in combination with other stains such as acid fuchsin (Masson A).

Masson trichrome staining is a multistep staining method commonly used to examine connective tissue, muscle, and other tissue components in histological specimens. It consists of several steps, with acid fuchsin and ponceau de xylidine applied to the tissue. Subsequent staining with other dyes, such as aniline blue or light green, results in a differentiated image of the various tissue components, with collagen fibers, smooth muscle, erythrocytes, and cytoplasmic structures stained differently.