Five isoinhibitors of chymotrypsin/elastase present in aqueous extracts of Ascaris were isolated. The reactive site in each isoinhibitor, the peptide bond that during encounter is positioned over the catalytic site in chymotrypsin, is Leu-Met. This bond was hydrolyzed by incubating intact isoinhibitors with 5-25 mol% chymotrypsin at pH 3.2 for 4-6 days (isoinhibitor 1) or 2.5-5 weeks (isoinhibitors 2-5). The reaction under these conditions did not proceed beyond 60% modified isoinhibitor (peptide bond hydrolyzed) and 40% intact inhibitor. The Leu-Met bond, hydrolyzed in modified isoinhibitor, can be resynthesized at pH 7.6 by incubating modified inhibitor with a stoichiometric amount of chymotrypsin bound to Sepharose CL-4B and then dissociating the complex in a kinetically controlled fashion with 5% trichloroacetic acid. The product, intact inhibitor, was obtained in greater than 80% yield. The site in the isoinhibitor that is positioned over the catalytic site in elastase during encounter is the same as for encounter with chymotrypsin. The Leu-Met bond hydrolyzed during encounter with elastase can be resynthesized by chymotrypsin. Chymotrypsin and elastase bind to the inhibitor at the same site.