This protein labeling kit comprises the far-red fluorescent dye CF660C in succinimidyl ester (SE) form and other necessary materials for labeling 3×1 mg antibody. CF660C is a cyanine-based dye engineered to be highly water-soluble without being excessively charged. As a result, CF660C-antibody conjugates have high signal-to-noise ratio in immunostaining. CF660C is several fold brighter and significantly more photostable than Alexa Fluor? 660. CF660C absorbs at 667 nm but can still be sufficiently excited by the 633 or 635 nm laser with emission at 685 nm in the borderline spectral region between far-red and near-IR fluorescence. Please also see CF660R, an exceptionally photostable 660 nm dye ideal for confocal microscopy.