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. 5-FAM is a single isomer. It is one of the most popular green fluorescent reagents used for in situ labeling peptides, proteins and nucleotides. It has also been used to prepare various small fluorescent molecules. 5-FAM-X, SE contains a seven-atom aminohexanoyl spacer (known as ‘X’) between the FAM fluorophore and the succinimidyl ester. The ‘X’ spacer separates the fluorophore from the biomolecule to which it is conjugated. The ‘X’ spacer may potentially reduce the quenching that typically occurs upon conjugation. We recommend this fluorescein derivative as the preferred dye for preparing fluoresceinated proteins when the fluorescence quenching of the labeling dyes by proteins is a serious problem.