Chicken ovomucoid (CO), an effective inhibitor of bovine trypsin, has a reactive site in each of three tandem domains. When CO was subjected to inhibition assay by the method of Green and Work, the second domain (CO Domain II) inhibited bovine trypsin but not TLE-Se, a trypsin-like enzyme from Streptomyces erythraeus, and the first domain (CO Domain I) inhibited neither bovine trypsin nor TLE-Se. However, when the interaction between CO and TLE-Se was analyzed by means of a Lineweaver-Burk plot, it was found that the ovomucoid exhibited competitive inhibition of the bacterial protease at pH 8.0 (Ki = 5.2 microM). In this case, the reactive-site peptide bonds of the first and second domains were specifically hydrolyzed. The isolated CO Domain I also exhibited competitive inhibition of TLE-Se (Ki = 3.1 microM), which specifically hydrolyzed its reactive-site peptide bond.