1. A completely inactive renin was isolated from hog kidney extract by affinity chromatography on pepstatin-aminohexyl-Sepharose and on an Affi-Gel Blue column. 2. This inactive renin had a molecular weight of 43 000 +/- 1500 as determined by gel filtration on Ultrogel AcA 44. Upon activation with trypsin, its molecular weight fell to 41 000 +/- 1400. 3. The inactive renin lacked the ability to bind renin-binding substance whereas trypsin-activated renin was able to bind the renin-binding protein and to form high-molecular-weight renin. 4. Chymotrypsin as well as trypsin could activate the inactive renin although less effectively. 5. The active renins generated from the inactive renin by the action of different proteolytic enzymes differed in their net charge, reflecting the specificities of the proteases used; the isoelectric points of the native, the trypsin-activated and the chymotrypsin-activated forms of renin occurred at pH 5.3, 5.1 and 4.8 respectively.