Tetrazine-sulfo-NHS Ester is water-soluble reagent that enables simple and efficient labeling of antibodies, proteins and any other primary amine-containing macromolecules with tetrazine moiety in 100% aqueous buffers. Tetrazine-sulfo-NHS Ester demonstrates exceptionally fast cycloaddition kinetics (up to 30 000 M-1 s-1) with trans-cyclooctene (TCO) as the dienophile, the fastest kinetics ever reported for any bioorthogonal reaction.