Hydrazide derivative of fluorescein. An amine-containing cell impermeant fluorescent probe that can be reversibly coupled to aldehydes and ketones on diverse molecules. This forms a Schiff base - which can be further reduced to generate stable amine derivatives by treatment with reducing agents such as sodium borohydride (NaBH4) or sodium cyanoborohydride (NaCNH3). Carboxylic acids of proteins and other water-soluble biopolymers can also be coupled to this fluorescein derivative in aqueous solution using water-soluble carbodiimides such as EDAC. Fluorescent tag for labeling of cell-surface functional groups (glycophorins) and many other diverse molecules, including DNA, RNA, polysaccharides, sialylated glycoproteins, carbonylated proteins, carbonyl derivatives, and N-acetylneuraminic acid. Useful for determining protein and peptide topology on the cell surface. This probe has been used for directly imaging of mucin-type O-linked glycoproteins within living cells, for imaging polysaccharides transported in living cells and for fluorescence labeling of short RNA by oxidation at the 3′-End. Spectral data: λex=492nm; λem=516nm (in 0.1 M Tris pH9.0).