In 22 dogs, the effect of fibrinolysin therapy of fresh intravascular clots was evaluated by serial angiograms and autopsies. The average time required for dissolution of such clots, when treated by systemic plasmin, was 4 hours and 45 minutes, while the average time necessary for lysis with local fibrinolytic therapy was 2 hours 10 minutes. Treatment of fresh venous and arterial clots did not disclose any significant difference in susceptibility to lysis. In 21 dogs, the effect of age of the clot on the effectiveness of plasmin therapy was studied in clots formed with serum and morrhuate. Clots treated within 4 days of formation were susceptible to dissolution, although clots treated after 3 days did not uniformly undergo complete lysis. The serum-induced clots were more amenable to fibrinolytic therapy than were those formed with morrhuate.