This protein labeling kit comprises the blue fluorescent dye CF350 in succinimidyl ester (SE) form and other necessary materials for labeling 3×1 mg antibody. CF350 is spectrally similar to the traditional blue fluorescent dye, AMCA. However, compared to AMCA, CF350 is more water-soluble, more photostable and at least 50% more fluorescent on proteins. CF350 is at least as bright as Alexa Fluor? 350 and thus can be a direct replacement for the latter for all applications.