Measurements of active and inactive renin have been made in a group of six anephric patients and in 30 normal healthy volunteers. Plasma renin activity (PRA) was detected in all of the anephric patients but values were below the normal range. A trypsin-activatable totally inactive renin was present in the plasma of all six patients. In three patients, levels of inactive renin were greater than that found for the mean value of the control group. Gel filtration studies of anephric plasma revealed a degree of heterogeneity of molecular forms of active high molecular weight (HMW) renin. However, an active form of approximately 65,000 daltons was usually present. Inactive renin, partially purified from anephric plasma had an apparent molecular weight (MW) of 59,000 daltons. Affinity chromatography with H.77 Sepharose was used to purify active renin from anephric and normal plasma following treatment with trypsin. The increased PRA in plasma from both groups following trypsin was associated with a 56,000 daltons form indicating little, if any, change in MW upon activation. The active HMW forms and the activated inactive renin were all totally inhibited by a renin-specific monoclonal antibody raised against human kidney renin.