We show that eight different serine proteinases--bovine chymotrypsins A and B, porcine pancreatic elastase I, proteinase K, Streptomyces griseus proteinases A and B, and subtilisins BPN' and Carlsberg--interact with turkey ovomucoid third domain at the same Leu18-Glu19 peptide bond, the reactive site of the inhibitor. Turkey ovomucoid third domain was converted to modified (the reactive site peptide bond hydrolyzed) form as documented by sequencing. Complexes of all eight enzymes both with virgin and with modified inhibitor were prepared. All 16 complexes were subjected to kinetically controlled dissociation, and all 16 produced predominantly virgin (greater than 90%) inhibitor, thus proving our point. During this investigation, we found that both alpha-chymotrypsin and especially S. griseus proteinase B convert virgin to modified turkey ovomucoid third domain, even in the pH range 1-2, a much lower pH than we expected. We have also measured rate constants kon and kon* for the association of virgin and modified turkey ovomucoid third domain with several serine proteinases. The kon/kon* ratio is 4.8 X 10(6) for chymotrypsin, but it is only 1.5 for subtilisin Carlsberg. A number of generalizations concerning reactive sites of protein proteinase inhibitor are proposed and discussed.