Biocytin hydrazide is a derivative of biocytin in which the typical amine group of the L-lysine residue is replaced with a reactive hydrazide functionality, thereby enabling specific chemical interactions. Biocytin hydrazide forms stable covalent bonds with aldehyde groups, making it a versatile tool for the targeted labeling of proteins, glycoproteins, and carbohydrates. Biocytin hydrazide is additionally used as an aldehyde-fixable cellular tracer in neuronal tracing studies, and it is also employed for the chemical modification of macromolecules within the fields of molecular biology and biochemistry, demonstrating broad research applicability.