Aminopeptidase N [AP-N, EC 3.4.11.2] is a Zn2+ dependent ectoenzyme that is anchored in the plasma membrane via a hydrophobic domain adjacent to a small cytoplasmic region at the NH2 terminus1,2). This enzyme is widely expressed by the brush border of the small intestine, synaptic cells of the central nervous system, and hematopoietic cells of myeloid lineage3). It has been postulated that AP-N performs multiple functions to regulate the action of hormones and neurotransmitters by inactivating such peptides at the cell surface4,5). Recently, this enzyme has been proved to be identical to the human myeloid plasma membrane glycoprotein CD13 which is expressed on human normal and malignant hematopoietic cells, and human malignant melanoma cells, but not normal melanocytes6,7). It has been indicated that AP-N is involved in the process to degrade and invade the extracellular matrix by tumor cells7-9). Furthermore, it has been demonstrated that the inhibition of AP-N by enzyme inhibitors or antibodies against AP-N is correlated with growth suppression of human tumor cells10). Thus, it is expected that the specific AP-N inhibitor may become a drug to suppress the metastasis or growth of certain cancers. We have already reported actinonin11), probestin12) and leuhistin13) as specific inhibitors of AP-N. We continued the screening for new AP-N inhibitors and discovered phebestin (Fig. 1) from the culture of Streptomyces sp. MJ716-m3. In this communication, we report the production, isolation, physico-chemical properties, structure determination and biological activities of the inhibitor.