This protein labeling kit comprises the green fluorescent dye CF555 in succinimidyl ester (SE) form and other necessary materials for labeling 3×1 mg antibody. CF555 is one of a series of CF dyes developed by Biotium for labeling proteins and other biomolecules. The dye is particular useful for labeling antibodies with results superior to those obtained with Alexa Fluor? 555 SE or Cy?3 SE. CF555 is brighter than the spectrally similar dye Cy?3.