Although FITC reagents have been more often used to prepare fluorescent bioconjugates, the low stability of FITC bioconjugates makes some researchers use amine-reactive succinimidyl esters of carboxyfluorescein (commonly called FAM) in bioconjugations. FAM reagents generate carboxamides that are more resistant to hydrolysis. We have shown that FAM reagents require less stringent reaction conditions, give better conjugation yields, and the resulted conjugates have superior stability. We noted that FITC labeled peptides tend to deteriorate more quickly than the corresponding FAM conjugates. 6-FAM is a single isomer of carboxyfluorescein. It is mainly used in sequencing of nucleic acids and labeling nucleotides.