1. Levels of serum inhibitor of plasminogen activation, anti-plasmin and plasminogen activator were measured in normal subjects and patients with active glomerulonephritis and chronic renal failure. 2. Patients with active glomerulonephritis all had grossly elevated levels of serum inhibitor of plasminogen activation and significant elevation of anti-plasmin. The majority of activator levels lay at the lower end of the normal range. 3. Patients with chronic renal failure had significantly elevated levels of serum inhibitor of plasminogen activation and anti-plasmin, but the changes were less marked than in those with active glomerulonephritis. Activator levels were consistently reduced. 4. The marked inhibition of fibrinolysis in active glomerulonephritis may be a factor in the persistence of glomerular fibrin and ultimately in perpetuation of renal damage. The changes in the fibrinolytic system in chronic renal failure may determine the development of the serosal exudates characteristic of that condition.