Glycogen-stimulated dog peritoneal leukocytes phagocytize particulate fibrin in a manner similar to that described for phagocytosis of bacteria. After phagocytosis the fibrin is solubilized, and degradation products, called here MISFI (molecules immunologically similar to fibrinogen), are released into the surrounding media. Whole, viable leukocytes are necessary; disrupted cells have little or no activity. This process of leukocytic ingestion and digestion of fibrin is called leukofibrinolysis. It is inhibited by many of the same substances that inhibit fibrinolysis. Serum, a potent fibrinolytic inhibitor, does not inhibit leukofibrinolysis.