This protein labeling kit comprises the blue fluorescent dye CF405M in succinimidyl ester (SE) form and other necessary materials for labeling 3×1 mg antibody. CF405M is spectrally similar to Pacific Blue dye. CF CF405M is as bright as Pacific Blue? dye but is significantly more photostable than the latter. An additional advantage of CF405M is that it has less spillover in the 525/50 green channel than Pacific Blue? dye due to narrower emission peak, making CF405M a superior choice in multi-color detection applications. We also offer CF405S, another 405 nm-excitable dye with an even shorter emission wavelength.