The cDNA of the rat carboxylesterase ES-3 encodes a polypeptide with 561 amino acid residues including a cleavable signal peptide at the N-terminus. The processed polypeptide shows over 90% sequence identity to mouse egasyn (ES-22); its calculated pI (5.5) matches the value determined for purified liver ES-3. The product expressed in COS cells migrates in native gels in the region of ES-3 and is similarly active on acetanilide. It is retained in the cells, as predicted from its C-terminus HTEL, and bears a single endo-H sensitive oligosaccharide chain. The nonglycosylated form expressed in the presence of tunicamycin is also intracellular, but substantially less active.