Crystal structure studies have shown that cleaved and intact serpins differ essentially in the topology of β-sheet A. This is five-stranded in the intact molecules and six-stranded after cleavage by insertion of strand s4A whose C-terminus has become free [Löbermann, H., Tokuoka, R., Deisenhofer, J. & Huber, R. (1984) J. Mol. Biol. 177, 531–556; Wright, T. H., Qian, H. X. & Huber, R. (1990) J. Mol. Biol. 213, 513–528]. The structural transition is accømpanied by changes in spectral properties and an increase in thermal stability. We show here that an Nα-acetyl-tetradecapeptide with the amino acid sequence of strand s4A, residues 345–358 of human α1-antitrypsin, associates with intact α1-antitrypsin and forms a stoichiometric complex with properties very similar to cleaved α1-antitrypsin. Complex generation has the characteristics of a folding process.