The serpin mechanism of action may resemble the "standard mechanism" described for small protein inhibitors of serine proteinases. Since these inhibitors are able to bind active site-modified target proteinases, we have investigated the interactions between two serpins and their 3,4-dichloroisocoumarin (DCI)-inactivated target proteinases. alpha 2-Antiplasmin and alpha 1-proteinase inhibitor bound stoichiometrically to DCI-inactivated chymotrypsin (EC 3.4.21.1) and DCI-inactivated human neutrophil elastase, respectively. Similar to active proteinases, the DCI-inactivated proteinases failed to bind complexes between serpins and synthetic reactive site loop peptides. Thus, the abilities of active and inactive proteinases to bind the serpins probably depend on the same structural characteristics. The thermodynamic stability of the alpha 2-antiplasmin-DCI/chymotrypsin and alpha 1-proteinase inhibitor-DCI/human neutrophil elastase complexes was similar to that of virgin serpins. However, in mouse plasma elimination studies the two complexes were removed rapidly from the circulation, suggesting that they have adopted the receptor recognized conformation. Consequently, cleavage of the reactive center peptide bond and formation of an inhibitor-acyl enzyme complex is neither obligatory to serpin-proteinase complex formation nor essential for the conformational change responsible for receptor mediated endocytosis.