Angiotensin I-converting enzyme was purified to electrophoretic homogeneity (12 units/mg) from bovine lung tissue and from human serum using an affinity gel described previously (Harris et al., (1981) Anal. Biochem. 111, 227-234). The isoelectric point (4.5), molecular weight (145 000), S20,W (8.1), amino acid composition and carbohydrate content of the lung enzyme are all similar to the values obtained for the human serum enzyme. The NH2-terminus of the lung enzyme (Ala) is different from that of the serum enzyme (Tyr) but the COOH-terminal sequences are identical (-Leu-Ser-OH). Pure bovine lung enzyme was reduced and carboxyamidomethylated with iodo (14C1) acetamide to the extent predicted by the number of cysteine residues. Since no radioactivity was incorporated into denatured enzyme that was not reduced, all of the cysteine residues must be in the form of disulfide bonds. Reverse-phase HPLC was used to separate peptides obtained from the lung enzyme after degradation with either trypsin or cyanogen bromide. The number of peptides resolved (42 after trypsin, 31 after cyanogen bromide), were only 20% fewer than the number predicted from the amino acid analysis and therefore the possibility that the converting enzyme (a single polypeptide chain) might be a fused dimer is excluded.