It was found that about 90% of the triacylglycerol lipase activity of a rat liver microsomal fraction was released by heparin treatment. The residual microsomal fraction contained about 95% of the esterase activity but little triacylglycerol lipase activity. Two kinds of esterase (esterases I and II) were purified from this residual fraction by DEAE-cellulose chromatography, hydroxyapatite chromatography, isoelectric focusing and Sephadex G-200 chromatography. The final preparations of esterases I and II, which were puri fied 70-and 140-fold, respectively, gave single protein bands on polyacrylamide gel and sodium dodecyl sulfate-gel electrophoreses. The molecular weights of esterases I and II were calculated to be about 70 000 and 160 000 by gel filtration of Sephadex G-200. Their isoelectric points were 5.82 and 6.32. Both esterases completely hydrolyzed short-chain triacylglycerols, such as tributyrylglycerol, but did not hydrolyze long-chain triacylglycerols. They preferentially hydrolyzed medium-chain-length 1-monoacylglycerols, such as 1-monocaprylylglycerol. Esterase I differed immunologically from esterase II and it was found to constitute about 30% of the total esterase activity in the microsomal fraction; esterase II constituted 50--60%. These results show the existence of two isozymes of esterase in the microsomal fraction.