The effect of fibrinolysis with Streptokinase and defibrination with Ancrod on the progression of established fibrin-related glomerular injury was assessed in rabbits developing anti-glomerular basement membrane antibody-induced glomerulonephritis. Untreated rabbits developed renal failure and a severe crescentic nephritis with prominent fibrin deposition after 5 days. Rabbits with established injury and glomerular fibrin deposition were treated with Streptokinase or Ancrod over the last 4 days of this model. Both treatments resulted in significant protection from loss of renal function and reduced crescent formation by day 5. Glomerular fibrin deposition was also significantly reduced by both agents, although Streptokinase produced a greater reduction than Ancrod. Two further groups of rabbits with advanced disease, were treated over the last two days of this model. Although treatment reduced glomerular fibrin deposition, no protection from loss of renal function was observed. These studies indicate that both treatments were effective, if used early, in preserving renal function in established fibrin related glomerulonephritis, but they did not effect the outcome of more advanced disease. Both agents prevented further glomerular fibrin deposition, although only early treatment with Streptokinase reduced glomerular fibrin to below pre-treatment levels.