Carboxylesterase (EC 3.1.1.1) has played an important part in our understanding of the toxicokinetic behaviour of the organophosphorus cholinesterase inhibitors. Carboxylesterases are a heterogeneous group of enzymes that can be separated on the basis of their isoelectric points and by their substrate-specificity. We have purified the isoenzyme (pI 5.8) present in greatest activity in rat lung to near homogeneity. The enzyme was purified by (NH4)2SO4 precipitation, gel filtration, chromatofocusing, separation on anion- and cation-exchangers and hydrophobic-interaction chromatography. The purified enzyme has a molecular mass of approx. 180 kDa with subunits of 60 kDa. The substrate and inhibitor specificities of the enzyme have been characterized. Edman degradation revealed the first 19 amino acid residues of the enzyme. The N-terminus was found to be tyrosine. Inhibition of the enzyme by organophosphorus compounds differed greatly from that of cholinesterases, despite the partial analogy at the N-terminal region.