1. Biochemical characteristics of a renin-like enzyme secreted by a pulmonary adenocarcinoma have been studied. A very high renin content was revealed by both enzymatic (10.4 Goldblatt units/g of tissue) and direct radioimmunoassay of immunoreactive renin (23 Goldblatt units/g of tissue).
2. The higher value by direct radioimmunoassay suggested the presence of an inactive form of the enzyme. Indeed 40–100% activation occurred with treatment by trypsin or pepsin or prolonged dialysis at pH 7.4 with or without prior acid dialysis. This neutral activation was completely abolished by a serine proteinase inhibitor.
3. A large fraction of the renin in this tumour is inactive. In comparison with other pro-hormones produced in tumours the findings support strongly the proposition that renin passes through a proenzyme step in synthesis.