Indocyanine green succinimidyl esters, near-infrared labeling dyes, were synthesized. These reagents were indocyanine green derivatives possessing N-hydroxysuccinimidyl groups capable of reacting with proteins. The absorption maximum of indocyanine green-labeled human IgG was 785 nm, and its fluorescent excitation and emission maxima were 768 nm and 807 nm, respectively.