2-Hydroxy-4-methylbenzenesulphonic acid ammonium is an impurity of Policresulen. Policresulen is a potent NS2B/NS3 protease inhibitor with an IC50 of 0.48 μg/mL. Policresulen effectively inhibits the replication of DENV2 virus in BHK-21 cells with an IC50 of 4.99 μg/mL. Policresulen acted as a competitive inhibitor of the protease, and slightly affected the protease stability.