||Many haemorrhagic complications have been attributed to increased intravascular fibrinolytic activity. The degradation products resulting from fibrinolysis have been shown to interfere in fibrin polymerization and have been observed in defective clot formation., Although considerable knowledge has accrued concerning the mechanism of filarinolysis and the resulting products, areas of controversy and intermittent obscurity remain» The study presented herein was conducted to resolve, if possible, existing discrepancies and characterize with certainty intermediate and end proteolytic products, An effort was made to correlate experimental findings with clinical disease states. The products of serial digestion of fibrinogen by plasmin were studied by immunodiffusion, immunoelectrophoresis and gel exclusion. Two products were observed. These products were identified as the D and E fragments, originally reported by Nussenweig et al. (1960). The D fragment was represented by a band located in the beta area of the electrophoretic field. This product was formed early in the digestion process and underwent no further degradation. The second product, a migratory band, probably representing a plasmin susceptible precursor of the E fragment, was initially observed within the locus of the D fragment. However, as digestion progressed the band migrated across the electrophoretic field to the prealbumin area, remaining in this position with exhaustive proteolysis. This band represented the plasmin resistant E fragment end product, Under certain experimental conditions the hydrolysis of fibrinogen into the E fragment may apparently be reversed. Serum samples from 196 hospitalized patients were studied for the presence of fibrinolytic intermediates and end products. Pathologic products were found in 10 of the 196 samples. Of these la samples, 8 contained D fragments, whereas Z samples contained both the D and E fragments. Clinical-pathologic correlation indicates that demonstrable fibrinolytic products were associated with injury to the hepatic, renal and vascular systems. The small molecular weight E fragment was encountered only in instances of renal failure.