Leupeptins, leupeptin Pr and leupeptin Ac, strongly inhibit proteolysis by plasmin, trypsin and papain, but do not inhibit proteolysis by α-chymotrypsin. The inhibition is competitive with substrates. The inhibitory effect on esterolysis by plasmin and trypsin is weaker than on proteolysis. The results with derivatives of leupeptins which contain carboxyl or alcohol instead of aldehyde and of di-n-butyl acetals of leupeptins indicate that the free aldehyde group plays a role in the activities. Leupeptins are absorbed orally and at least about 25 % is excreted in urine. Oral administration of leupeptins exhibited an anti-inflammatory effect on edema. Leupeptins inhibited thrombokinase reaction and coagulation of blood of human and rabbit. Type of inhibition was different from heparin. Coagulation of blood of rats and dogs are not inhibited. The effects of leupeptins on thrombokinase, thrombin, plasmin, trypsin, papain, kallikrein and α-chymotrypsin were compared with those of e-aminocaproic acid, trans-4-aminomethylcyclohexanecarboxylic acid, soybean trypsin inhibitor and trasylol.