The interaction of duodenase, a new serine protease from a small group of Janus-faced proteases, with serpins, alpha 1-protease inhibitor (alpha 1-PI) and antichymotrypsin (ACT) from human blood serum, was studied. The stoichiometry of the inhibition process was found to be 1.2 and 1.3 mol/mol for alpha 1-PI and ACT, respectively. The presence of a stable enzyme-inhibitory complex duodenase-alpha 1-PI was confirmed by SDS-PAGE. No formation of the duodenase-ACT complex was demonstrated; instead, the band of the cleaved inhibitor was indicated upon the ACT hydrolysis. The suicide mechanism of the duodenase interaction with the human blood serpins was proved. The association rate constants (Ka, M-1 s-1) were 2.4 +/- 0.3 x 10(5) for alpha 1-PI and 3.0 +/- 0.4 x 10(5) for ACT. These results indicate the possibility of the regulation of duodenase activity by endogenous serpins. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2003, vol. 29, no. 6; see also http://www.maik.ru.