Chicken, sheep, and horse liver carboxylesterases have been purified by procedures involving ammonium sulfate fractionation, ion-exchange chromatography and gel filtration on Sephadex. The actual yields of the procedures described were as follows: chicken, 1 g from 2 kg of liver powder (chloroform–acetone); sheep, 200 mg from 400 g of powder (chloroform–acetone); horse, 230 mg from 800 g of powder (acetone). The purified enzymes are free of non-carboxylesterase protein as shown by gel electrophoresis, although they do contain electrophoretic variants. The equivalent weight of the chicken enzyme is 67 000 based on titration with p-nitrophenyl diethyl phosphate or bis(p-nitrophenyl) phosphate, whereas those of the sheep and horse enzymes are ~69 500 and ~70 000, respectively, based on titration with p-nitrophenyl dimethylcarbamate.