Hexa-D-arginine (Furin Inhibitor II) is a stable furin inhibitor with Ki values 106 nM, 580 nM and 13.2 μM for furin, PACE4 and prohormone convertase-1 (PC1), respectively. Hexa-D-arginine blocks Pseudomonas exotoxin A and anthrax toxins toxicity in vitro and in vivo.