This protein labeling kit comprises the red fluorescent dye CF568 in succinimidyl ester (SE) form and other necessary materials for labeling 3×1 mg antibody. CF568 has an excitation spectrum optimally matching the 568 nm line of the Ar-Kr mixed-gas laser. Antibody conjugates of the dye are much brighter than those of Alexa Fluor? 568. In addition, the photostability of CF568 is superior to that of Alexa Fluor? 568, making CF568 a much better choice for demanding applications, such as confocal microscopy and single molecule imaging.