mal-sac-HNSA, an often used freely water-soluble, heterobifunctional crosslinking reagent for conjugation of sulfhydryl (cysteine)-containing peptides to carrier proteins. The reaction of mal-sac-HNSA with amino groups can be monitored spectrophotometrically due to the concomitant release of the dianion of 1-hydroxy-2-nitro-4-benzenesulfonic acid.