For expression of Bacillus stearothermophilus NCIB 8924 leucine aminopeptidase II (LAP II) in Escherichia coli regulated by a T5 promoter, the gene was amplified by polymerase chain reaction and cloned into expression vector pQE-32 to generate pQE-LAPII. The His(6)-tagged enzyme was overexpressed in IPTG-induced E. coli M15 (pQE-LAPII) as a soluble protein and was purified to homogeneity by nickel-chelate chromatography to a specific activity of 425 U/mg protein with a final yield of 76%. The subunit molecular mass of the purified protein was estimated to be 44.5 kDa by SDS-PAGE. The temperature and pH optima for the purified protein were 60degreesC and 8.0, respectively. Under optimal condition, the purified enzyme showed a marked preference for Leu-p-nitroanilide, followed by Arg- and Lys-derivatives. The HiS(6)-tagged enzyme was stimulated by Co2+ ions, but was strongly inhibited by Cu2+ and Hg2+ and by the chelating agents, DTT and EDTA. The EDTA-treated enzyme could be reactivated with Co2+ ions, indicating that it is a cobalt-dependent exopeptidase. Taking the biochemical characteristics together, we found that the recombinant LAP II exhibits no important differences from those properties described for the native enzyme.