Preliminary studies indicate that 3-butylthio-1,1,1-trifluoropropan-2-one (BTFP) is a potent, competitive inhibitor of pigeon serum alpha-naphthyl acetate esterase (alpha-NAE); I50, 9.5 nm.The synthesis of an affinity resin with 3-(4-mercaptobutylthio)-1,1,1-trifluoropropan-2-one (MBTFP) as the ligand is reported; alpha-NAE from serum bound strongly to this. To remove protein from the resin, washing with 3-octylthio-1,1,1-trifluoropropan-2-one (OTFP) and subsequent boiling in aqueous sodium dodecylsulfate (SDS) were employed. Recovered protein run on SDS-PAGE showed a single major protein band at 58-60 kDa which agrees well with previous determinations of the molecular weights of carboxylesterases.