cr,-Macroglobulin (QM) forms proteolytically inactive but esterolytically active complexes with trypsin and thrombin 1-3. The esterolytic activity of the trypsina, M complex is highly resistant to trypsin inhibitors such as soy-bean trypsin inhibitor, STI, and a,-antitrypsin114. a, M is known to be a potent inhibitor of the proteolytic activity of plasmins. Trypsin and plasmin seem to be bound to the same binding site of OZ, M~. Therefore it was considered of interest to compare the esterolytic activity of plasmin with that of the plasmin-cr, M complex. A purified u, M preparation was obtained from pooled human serum by precipitation of the low-density lipoproteins with dextrane sulphate7 and by subsequent gel filtration in Sephadex G-zoo. 400,~ l of the preparation (containing about 0.15 nmole a, M) was incubated at 37O with increasing amounts of autoactivated human plasmin (Kabi, grade A), which had been dialysed overnight to remove the glycerol added to stabilize the preparation. After 5 min the esterolytic activity was determined with p-toluene sulfonyl-L-arginine methyl ester HCl (TAME) as substrates. The activity was found to increase linearly with the amount of plasmin added. With the substrate concentration used in the incubation mixture the specific activity was about 9096 of that found when the same amounts of plasmin were incubated without the macroglobulin fraction (Fig. I). In another experiment the cc, M preparation was incubated