We have cloned and sequenced a carboxylesterase from rat liver and purified the corresponding protein from rat blood. The cDNA encodes the entire mature serum esterase protein. It is apparently identical to cDNAs cloned from rat liver by several groups (Long, R. M., Satoh, H., Martin, B. M., Kimura, S., Gonzales, F. J., and Pohl, L. R. (1988) Biochem. Biophys. Res. Commun. 156, 866-873; Takagi, Y., Morohashi, K.-i., Kawabata, S.-i., Go, M., and Omura, T. (1988) J. Biochem. (Tokyo) 104, 801-806; and Robbi, M., and Beaufay, H. (1992) Biochem. Biophys. Res. Commun. 183, 836-841). However, the identification of the protein encoded by this cDNA has not been previously reported. The COOH-terminal -TEHT sequence found in the rat serum carboxylesterase does not possess retention properties and is therefore responsible for its secretion and presence in the circulation. The rat serum carboxylesterase was purified to apparent homogeneity by affinity chromatography on immobilized antibody to rat liver microsomal acyl-CoA thioesterase followed by ion exchange chromatography. The purified protein, with a M(r) of approximately 70,000, was cleaved in situ in a polyacrylamide gel with trypsin, and two peptides were isolated and sequenced. Sequence analysis showed that both peptides were identical only to the corresponding deduced amino acid sequence of the cloned cDNA. Antibodies raised to the COOH-terminal amino acid sequence deduced from the cDNA cross-reacted with the purified rat serum carboxylesterase. Changes in serum esterase activity levels followed changes in protein mass in rat serum and changes in liver mRNA levels in response to various nutritional conditions while total liver esterase activity was essentially unchanged. The above experiments confirm the identity of the protein isolated from rat sera with the cDNA cloned from rat liver and suggest a function for the serum esterase in lipid metabolism.