1. Human angiotensin-converting enzyme from kidney, lung, small intestine, plasma and urine was found to be immunologically indistinguishable by double immunodiffusion and inhibition of enzyme activity with specific anti-(human kidney enzyme) antibody.
2. Partial inhibition of the enzyme activities from hog, dog and rat sera by the antibody was found, but no inhibition was observed in the enzymes from sheep and bovine sera.
3. Human angiotensin-converting enzyme was located in two types of cells by an immunofluorescent technique using the purified antibody and fluorescein-labelled anti-(rabbit immunoglobulin) antibody. These were the vascular endothelial cells of alveolar capillaries and of pulmonary interstitial small vessels, and the epithelial cells of proximal tubules in the kidney and of mucous membrane in the small intestine.