The classical Bowman-Birk soybean proteinase inhibitor was modified by hydroxyethylstarch. The modified inhibitor retained the capacity for simultaneous binding of trypsin and human leukocyte elastase. The inhibition constants, K-i, of bovine trypsin, alpha-chymotrypsin and human leukocyte elastase (HLE) increased not more than 10-, 1.5-, and 20-fold, respectively, on modification of the inhibitor. The less effective inhibition is presumably due to the steric hindrance brought about by the conjugation with polysaccharide molecules. The results obtained indicate the pronounced structure differences of the binding regions for trypsin and chymotrypsin/leukocyte elastase in the modified preparation.