This protein labeling kit comprises the blue fluorescent dye CF405S in succinimidyl ester (SE) form and other necessary materials for labeling 3×1 mg antibody. CF405S has an absorption peak wavelength that nearly coincides with the 405 nm blue diode laser line. In addition, the emission peak wavelength of the dye well centers within the blue detection window of BD flow cytometers. As a result, flow cytometry analysis using CF405S dye results in much brighter signal than that using Alexa Fluor? 405.