Aminopeptidase N (AP-N, or aminopeptidase M, EC 3.4.ll.2) is a member of a family of membrane-bound metallopeptidases with wide distribution on the surfaces of diverse cell types.X) This enzyme plays the part of an enzyme which inactivates enkephalins, the pentapetides with morphine-like activities in cerebral membranes.2) In recent years, the human myeloid plasma membrane glycoprotein CD13 has been proved to be identical to AP-N,3) and it is expressed in human myeloid leukemia cell lines4) and malignant mesenchymal tumors.5) Saiki et al.6) and Menrad et al.1] have found aminopeptidase N to play important roles in the invasion of metastatic tumors in vitro. Recently, Ino et al.8) also suggested that AP-N inhibitors such as bestatin and actinonin could become new drugs for cancer treatment, not as biological response modifiers (BRM), but as anti-cancer chemicals exerting direct suppressive action on the growth or metastasis of certain cancers. Thus, the specific inhibition of AP-N could be a new approach to suppress the growth or metastasis of cancer. During the screening for new inhibitors of aminopeptidase N, we obtained new substances MR-387A and B (Fig. 1) from the culture broth of Streptomyces neyagawaensis SL-387. In this communication, we report the isolation, physico-chemical properties, structure elucidation and biological activities of the inhibitors.