Despite the availability of alternative amine-reactive fluorescein derivatives that yield conjugates with superior stability and comparable spectra, fluorescein isothiocyanate (FITC) remains one of the most popular fluorescent labeling reagents, probably due to the low cost of the material. 5-FITC and 6-FITC have very similar absorption and fluorescence spectra. However, the isomers may differ in their binding and reactivities to proteins, and the conjugates may elute under different chromatographic conditions or migrate differently in electrophoretic gels. Thus, we offer highly purified single isomers. It appears that 5-FITC is more widely used than the 6-FITC isomer. FITC reagents are prominently used to label proteins. In addition, FITC has also been used to label peptides, oligonucleotides and other small organic ligands. A collection of recent applications is listed in the following references.