Leuhistin has been isolated from the culture broth of Bacillus laterosporus BMI156-14F1 as part of a program designed to find microorganism-produced inhibitors of aminopeptidase M (AP-M). It was purified by use of column chromatography on Sepabeads SP206, Amberlite IRC-50, MCI gel CHP-20P and Sephadex G-10 and then isolated as colorless needles. Leuhistin inhibits AP-M strongly and it also inhibits AP-A and AP-B weakly. It is competitive with the substrate, and the inhibition constant (Ki) was 2.3 x 10(-7) M.