A novel inhibitor of prolyl endopeptidase, the propeptin analog propeptin-2 (C 105 H 130 N 24 O 24 ), missing two amino acid residues from the propeptin C-terminus was isolated from the fermentation broth of propeptinproducing Microbispora sp. SNA-115 grown using a large inoculum. It shows the same enzyme inhibition activity as propeptin against prolyl endopeptidase (KiĎ1.5 m M), but its antimicrobial activity against Mycobacterium phlei is more than 100-fold lower.Keywords propeptin, Microbispora sp., Mycobacterium spp., prolyl endopeptidase Prolyl endopeptidase (PEP; post-proline cleaving enzyme; official name: prolyl oligopeptidase; EC 3.4.21.26) cleaves peptide bonds at the carboxyl side of proline residues and hydrolyzes biologically active peptides containing proline, such as vasopressin and substance P [1,2]. It may also be involved in diseases such as Alzheimer's disease and bipolar affective disorder [3Ďł5]. Meanwhile, effective antituberculosis drugs with novel structures and mechanisms of action have not been developed in over 30 years. Moreover, the increase in Human Immunodeficiency Virus (HIV) and cancer patients has resulted in the spread of opportunistic bacteria, such as the Mycobacterium avium complex (MAC).Propeptin ( Fig. 1(a)