Subsites in the S2-S4 region were identified in human plasma kallikrein. Kinetic constants (kcat., Km) were determined for a series of seven extended N-aminoacyl-L-arginine methyl esters based on the C-terminal sequence of bradykinin (-Pro-Phe-Arg) or (Gly)n-Arg. The rate-limiting step for the enzyme-catalysed reaction was found to be deacylation of the enzyme. It was possible to infer that hydrogen-bonded interactions occur between substrate and the S2-S4 region of kallikrein. Insertion of L-phenylalanine at residue P2 demonstrates that there is also a hydrophobic interaction with subsite S2, which stabilizes the enzyme-substrate complex. The strong interaction demonstrated between L-proline at residue P3 and subsite S3 is of greatest importance in the selectivity of human plasma kallikrein. The purification of kallikrein from Cohn fraction IV of human plasma is described making use of endogenous Factor XIIf to activate the prekallikrein. Kallikreins I (Mr 91 000) and II (Mr 85 000) were purified 170- and 110-fold respectively. Kallikrein I was used for the kinetic work.