A peptidase acting on Leu-Gly-Gly and Leu-Tyr at pH 8 to 10 was purified about 670-fold from germinated grains of barley (Hordeum vulgare L.). Gel electrophoretic analyses indicated a purity of about 90%. The purified enzyme is remarkably similar to mammalian leucine aminopeptidases (EC 184.108.40.206) both in chemical and in enzymatic properties. It has a sedimentation constant of 12.7S and a molecular weight of about 260,000. The enzyme has a high activity on leucine amide and di- and tripeptides with N-terminal leucine or methionine; leucyl-beta-naphthylamide, in contrast, is hydrolyzed very slowly. The enzyme also liberates N-terminal amino acids from the insulin B chain. The pH optima for the hydrolysis of different substrates depend on the buffers used; highest reaction rates are generally obtained at pH 8.5 to 10.5. Mg(2+) and Mn(2+) ions stabilize (and probably activate) the enzyme. In contrast to mammalian leucine aminopeptidases, the barley enzyme is inactivated in the absence of reducing sulfydryl compounds.