Serial assays of plasma fibrinolytic activity, euglobulin lysis activity, fibrinogen, and plasminogen were carried out in 13 children with acute post-streptococcal glomerulonephritis (APGN). Observations were started 2 to 16 days after the apparent onset and were continued until the clinical recovery. A marked increase in plasma overall fibrinolytic activity was found in 12 out of 13 patients during a relatively early stage of the disease. The fibrinolytic activity was due to an increase in plasma activators level as it was shown by the elevated activity of the euglobulin fraction of the plasma. Fibrinogen was increased in most patients and plasminogen level was lowered in some of them. The increase in plasma fibrinolytic activity was no longer detectable during the subsiding phase of the disease. Plasma fibrinolytic hyperactivity probably relates with the removal of fibrin from the glomeruli.