AbstractIn histochemistry, the terms Schiff's reagent and leucofuchsin are sometimes used as synonyms. Though these compounds are derived from the same dyes they are chemically very different. In dyes of the fuchsin group, three aniline or ortho-toluidine rings are attached by single bonds to a central carbon atom. The fourth carbon valency is unoccupied and allows passage of mobileπ-electrons from ring to ring. These molecular orbitals cause the color of triphenylmethyl dyes. In leuco Pararosanilin (leucofuchsin), a hydrogen atom is firmly attached at the fourth binding site of the carbon atom and prevents passage ofπ-electrons; therefore, the compound is decolorized. Leuco Pararosanilin cannot be recolorized under the conditions of the Feulgen or PAS reaction. The colorless carbinol derivative contains a hydroxy group instead of the hydrogen at the central carbon atom. In Schiff's reagent, a sulfonic acid group is loosely bound at the free valency of the carbon atom. This sulfonic acid group is removed during or after reaction of the molecules with aldehyde groups and the color of the dye is restored. Sinceleuco Pararosanilin cannot be substituted for Schiff s reagent and is chemically very different, the use of the term leucofuchsin as a synonym for Schiff's reagent is incorrect and should be discontinued.