This protein labeling kit comprises the far-red fluorescent dye CF™660R in succinimidyl ester (SE) form and other necessary materials for labeling 3×1 mg antibody. CF™660R is a bright and exceptionally photostable dye that absorbs at 663 nm and emits fluorescence at 682 nm in the borderline spectral region between far-red and near-IR. CF™660R is ideal for confocal microscopy, where it can add a long wavelength color in multi-color imaging.