AcidiFluor ORANGE-NHS (succinimidyl ester) is pH probe “AcidiFluor ORANGE” which bonds amino groups of protein etc., and not need condensing agent. Not only antibody or protein but also oligonucleotide which terminal amination can be labeled by NHS. The contents of this kit cover from labeling to purification. A portion of AcidiFluor ORANGE-NHS is included most appropriate amount for labeling 100 μg of IgG.