The amine-reactive AMCA-X SE (6-((7-amino-4-methylcoumarin- 3-acetyl)amino)hexanoic acid, succinimidyl ester) and its conjugates yield blue-fluorescence (approximate excitation/emission maxima ~353/442) that is widely used to label antibodies, proteins and small drug molecules. Because its fluorescence may not be as bright as that of other dyes or may be obscured by autofluorescence, AMCA is only recommended for use with highly abundant targets. The AMCA-protein conjugates are suitable for immunofluorescent staining, in situ hybridization, flow cytometry and other biological applications. The kit has all essential components for performing the conjugation reaction and for purifying the AMCA-protein conjugates.