Wehave reported that specific inhibitors of various enzymes located on the cellular membrane, had important effects on cellular functions1^. These inhibitors included esterastin which inhibited esterase and suppressed immune responses45 (as well as) ebelactones which inhibited esterase but enhanced immuneresponses55. Continued screening for esterase inhibitors has resulted in the discovery of another inhibitor which we have named valilactone. Valilactone has no effect on immune responses. In this paper, we report on the isolation and characteri-zation of valilactone.