We demonstrated that activities of aminopeptidases, alkaline phosphatase and esterase were located on the surface of various types of animal cells1,2). Glycosyl-transferase and hydrolytic enzymes have also been reported to be localized on the surface of mammalian cells35). It is reasonable to assume that the enzyme activities of the cell surface play a key role in cell functions. In order to elucidate the biological role of these enzymes in various functions of the cells, we have searched for inhibitors against these enzymes and have succeeded in obtaining several of them from culture filtrates of actinomycetes. They are bestatin, forphenicine and esterastin610).These inhibitors were proved to bind to the cellular surface and modify the functions of immune response-participating cells11-13)We searched for an inhibitor of aminopeptidase A and found a new compound named amastatin. In this paper, we report the isolation and characterization of amastatin.