Endo-oligopeptidase A known to hydrolyse the Phe5-Ser6 bond of bradykinin and the Arg8-Arg9 bond of neurotensin has been shown to produce, by a single cleavage, leucine5-enkephalin from small prodynorphin derived enkephalin-containing peptides. The specificity constants (kcat/km) obtained for the hydrolysis of bradykinin, neurotensin and dynorphin B are of the same order, suggesting that the substrate amino acid sequence is not the only factor determining the cleavage site of this enzyme.