Lung converting enzyme (LCE) was extracted from normal human lung obtained at autopsy. The specificity of the enzyme preparation was confirmed in vitro by incubation with Angiotensin I (AI) and paper chromatographic identification of reaction products. A method was developed for assay of the enzyme preparation in human serum using tritiated Angiotensin I (H3AI) as substrate. Enzyme activity is quantitated by scintillation counting the radioactive end product tritiated dipeptide histadyl- Ieucine (H3His-leu). Serum from the authors and pooled serum from the hospital laboratory caused less than 3.5% conversion of H3AIto H3His-leu. Serum from two patients with severe lung damage caused a maximum 20% conversion of H3AIto H3His-leu. Percent conversion in these patients correlated with clinical and laboratory evidence of lung dysfunction.