We have attached two BODIPY FL fluorophores to the amino groups of the disulfide-containing amino acid cystine to create this reagent for reversible thiol-specific labeling of thiolated oligonucleotides, proteins and cells. BODIPY FL L-cystine is virtually nonfluorescent due to interactions between the two fluorophores; however, thiol-specific exchange with thiolated biomolecules occurs to form mixed disulfides, resulting in green fluorescence.