Leupeptin hemisulfate (Ac-Leu-Leu-argininal) is a reversible inhibitor of serine and cysteine endopeptidases. Leupeptin hemisulfate is capable of inhibiting plasmin, trypsin, papain, kallikrein, calpain and cathepsin B. No inhibition has been found with pepsin, cathepsins A and D, elastase, renin or chymotrypsin.