Renin in the mouse kidney has a molecular weight of 40 000.

1. Mouse kidney was homogenized in a mixture of serine-metallo- and thiol-enzyme inhibitors. The homogenate proteins were separated with respect to size and charge by gel filtration, agarose electrophoresis and polyacrylamide-gel electrophoresis. 2. Renin was localized by its enzymatic activity by using the antibody trapping radio-immunoassay for angiotensin I, before and after acid treatment and limited proteolysis. Renin was also localized by its antigenic properties by using antirenin antibodies elicited against pure mouse submaxillary renin. The antibody cross-reacted fully with mouse kidney renin and with high-molecular-weight renin forms in mouse plasma. 3. In the kidney only fully enzymically active 40 000 renin could be detected enzymically and antigenically. No high-molecular-weight renin or inactive renin was demonstrable. 4. Two electrophoretically different renin forms were seen in accordance with renin being a glycoprotein. They were both fully enzymically active with identical specific enzymatic activities. 5. The mouse kidney renin had a specific enzymatic activity identical with that of pure mouse submaxillary renin, being 0.4 Goldblatt unit/microgram.