3-Azidopropionic Acid Sulfo-NHS Ester is water-soluble reagent that enables simple and efficient labeling of antibodies, proteins and any other primary amine-containing macromolecules with azido group in 100% aqueous buffers. 3-Azidopropionic Acid Sulfo-NHS Ester is a reagent of choice for applications that cannot tolerate organic co-solvents or are complicated by their inclusion. Specific labeling of cell surface proteins is another common application for these uniquely water-soluble and membrane impermeable reagents.