Elastase is a proteolytic enzyme obtained from pig pancreas, which shows a high degree of amino acid sequence homology with other serine proteinases, including bovine trypsin and chymotrypsin (Hartley, this volume, p. 77). It consists of a single polypeptide chain of 240 residues, which corresponds to the single polypeptide chain of trypsin, and the B and C chains of chymotrypsin. Elastase possesses a common catalytic mechanism with these enzymes but differs from them in its substrate specificity, cleaving peptide bonds on the carboxyl terminal side of amino acid residues lacking charged or aromatic side chains (Naughton & Sanger 1961). Several workers have suggested that homologous enzymes with common catalytic mechanisms have very similar tertiary structures. This prediction was supported by Blow and his co-workers, who found that the two disulphide bridges present in trypsin, but absent in chymotrypsin, could be built into the molecular model of a-chymotrypsin with little or no distortion of the polypeptide chain (Sigler, Blow, Matthews & Henderson 1968), and by Hartley (this volume, p. 77) who has shown that the trypsin and elastase side chains can be substituted for those present in a skeletal molecular model of a-chymotrypsin with no gross distortions of the polypeptide chain.