Esterastin is a potent inhibitor of lysosomal acid lipase. Esterastin inhibited the enzyme activity very strongly (IC50, about 80 nM). The inhibition of acid lipase by Esterastin was competitive with respect to the substrate and the inhibition constant for Esterastin was 90 nM. Esterastin was less inhibitory to other lipolytic enzymes, such as pancreatic lipase and carboxylesterase. Thus, Esterastin is a potent new inhibitor of lysosomal acid lipase.