The interaction of the human plasma protein, alpha-1-antitrypsin, with porcine pancreatic elastase was studied by isolating and characterizing their reaction products. Native alpha-1-antitrypsin has a mass ratio (Mr) of 54,000, an amino-terminal glx, and a carboxy-terminal lys residue. The elastase used has an Mr of 26,400 and an amino-terminal val residue. When the two proteins are combined at inhibitor excess, two major products result. One of the products is a complex of the enzyme and inhibitor with amino-terminal ser and val residues, which indicates that a peptide has been removed from the amino-terminal end of the inhibitor. The second product is a modified form of alpha-1-antitrypsin with an Mr of 51,300, an aminoterminal glx residue and a carboxy-terminal thr-leu dipeptide. It has no inhibitory activity against elastase. The components of the isolated complex can be split at high pH in the presence of diisopropyl fluorophosphate, which results in a catalytically inactive enzyme with the same Mr and amino-terminal residue as the native enzyme, and a large fragment of alpha-1-antitrypsin (alpha-1-antitrypsin(*)). This fragment has an Mr of 50,100, an amino-terminal ser residue and a carboxy-terminal thr-leu dipeptide. Based on these data, the following hypothesis is proposed. Elastase can attack alpha-1-antitrypsin at either of two major sites. If it attacks first at the carboxy side of the thr-leu dipeptide, located in the carboxy-terminal portion of the inhibitor, the alpha-1-antitrypsin is cleaved into two fragments with loss of inhibitory activity and absence of complex formation. If, however, the elastase first attacks an x-ser bond near the amino-terminal end of the inhibitor, the elastase then reacts with alpha-1-antitrypsin at the same leu moiety to form a stable complex with complete inhibition of the enzyme.