SUMMARY Inactive renin and active renin from human kidney and human plasma were prepared in highly purified forms by three steps of chromatography on Octyl-Sepharose, Immunoafflnity chromatography, and pepstatin-amino hexyl Sepharose CL-4B. The inactive renin and active renin from human kidney had molecular weights of 51,000 and 44,000 as measured by a calibrated gel filtration column run with internal molecular weight standards. Molecular weights of plasma Inactive renin and active renin were 56,000 and 51,000 respectively. Both inactive and active renins were found to be heterogeneous, consisting of several components with different isoelectric points. Renal inactive renin has higher pi values of 6.40,6.10, 5.90,5.61, and 5.40. Renal active renin has pi values of 5.73,5.40,5.25, and 5.13. The pi values of plasma inactive renin were 6.37, 6.08, 5.77, 536, and 5.25; of plasma active renin, 5.68, 5.40, 5J3, and 5.25. Trypsin activation and plasmin activation of plasma inactive renin produced an active enzyme with similar molecular weight but lower pi values. Acid activation of inactive renin did not change the molecular weight and pi values.