The 1,839-base pair complementary DNA (cDNA) for rat lung carboxylesterase was cloned by reverse transcriptase polymerase chain reaction from total rat lung RNA using specific primers derived from the 5' and 3' untranslated regions of rat hepatic cholesteryl ester hydrolase (CEH). The unique cDNA was sequenced and found to be similar to hepatic CEH, pI 6.1 esterase, and hydrolase A. In Northern blot analysis, the cDNA hybridized with a single band from lung messenger RNA (mRNA). The 1.7-kb coding sequence, predicting a 62-kD protein, was transfected into COS-7 cells and Chinese hamster ovary (CHO) cells. Expression in COS-7 and CHO cells was accompanied by 4- and 3.2-fold increases in carboxylesterase activity (hydrolysis of p-nitrophenyl acetate), respectively. Unlike the hepatic CEH, the expressed lung carboxylesterase described here did not hydrolyze cholesterol esters. In situ hybridization experiments localized the lung carboxylesterase mRNA to the airway epithelium. The organophosphorus compound phosphoric acid diethyl 4-nitrophenyl ester, paraoxon, completely inhibited this lung carboxylesterase, placing it in the family of B esterases by this criterion.