Purpose. In this work, the alacepril thiolesterase, which catalyzes the hydrolyzing reaction of the thiolester linkage in alacepril and the conversion from alacepril to deacetylalacepril, was purified from rat liver cytosol and characterized. Methods. A purification procedure for the thiolesterase consisted of ammonium sulfate fractionation and chromatographies with phenyl Sepharose CL-4B, Q Sepharose FF, ceramic hydroxylapatite, and phenyl Sepharose HP. The thiolesterase activity was assayed for alacepril as a substrate and the reaction product, deacetylalacepril, was measured using high-performance liquid chromatography. Results. The purified thiolesterase is heterodimeric with a molecular mass of 29 and 36 kDa subunits as estimated by sodium dodecyl sulfate -polyacrylamide gel electrophoresis. N-terminal amino acid sequence of these subunits reveals that the thiolesterase is identical to sialic acid 9-O-acetylesterase. The thiolesterase hydrolyzes not only the thiolester bond in alacepril, spironolactone, and acetyl coenzyme A but also the carboxylester bond in α-naphtyl acetate. The alacepril thiolestrase activity is competitively inhibited by α-naphtyl acetate. Conclusion. The thiolesterase, i.e., sialic acid 9-O-acetylesterase, seems to be involved in the metabolism of certain drugs such as alacepril and spironolactone. However, drugs having ester-type and amide-type linkages, for example dilazep, aniracetam, and benazepril, are not substrates for the thiolestrase.