In culture filtrate of Streptomyces hygroscopicus a producer of polyketide antibiotics, a leucine aminopeptidase and its autogenous inhibitor were detected. The leucine aminopeptidase was purified 4573-fold with yield of 82% by combination of ion exchange and hydrophobic chromatography. The enzyme is monomeric with a molecular mass of 51 kDa determined by gel chromatography and 67 kDa determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Optimal activity was at pH 8.0 and 40degreesC. The pI of leucine aminopeptidase is 8.2. The enzyme is strongly inhibited by 1,10-phenantroline, amastatin and dithiothreitol. Atomic absorption spectrometry indicated 2 mols of ion zinc per mol of enzyme. The enzyme is stable at up to 70degreesC. Leucine aminopeptidase prefers leucine and methionine as N-terminal amino acids. Activity of leucine aminopeptidase is strongly modulated by an autogenous low-molecular weight inhibitor during fermentation, especially during periods of intensive antibiotic production.