1. Two high-molecular-weight forms of renin (molecular weights 800 000 and 70 000) are present in mouse plasma.
2. The 800 000 form could be activated and converted into the fully active 40 000 form, by acid or limited proteolysis. The 70 000 form was activated without change in molecular weight.
3. In addition to its enzymic activity, renin was measured by a direct radioimmunoassay, which revealed that the current acid treatment of plasma did not activate all the renin present.
4. Renin is stored as fully active 40 000 renin, with a specific enzymic reactivity of 0·4 × 10−3 GU ng−1, in the submaxillary gland of mice.
5. Pure 125I-labelled 40 000 submaxillary renin did not bind to plasma proteins. However, by changing the tertiary structure of renin, it was bound to some of the plasma protease inhibitors; α2-macroglobulin, inter-α-trypsin inhibitor and α2-antithrombin. It was also bound to α1- and β1-lipoprotein, albumin and an unidentified plasma protein. No binding was seen to more than 50 other studied plasma proteins.