Recently we demonstrated that activities of aminopeptidases and alkaline phosphatase were located on the surface of various kinds of animal cells. The activity of esterase was also located on the cellular surface. These enzymes may play an indispensable role in various functions of cells. This has been demonstrated by our experiments with bestatin, amastatin and forphenicine, specific inhibitors of aminopeptidases and alkaline phosphatase, respectively. These inhibitors enhanced immune responses. In the continuation of the study of enzyme inhibitors, we screened actinomycetes culture filtrates for the activity against pancreatic esterase and found a new compound, esterastin. In this communication, the isolation and characterization of esterastin is reported.