In vitro data have indicated that plasmin-mediated lysis of crosslinked (XL) fibrin in vivo yields only one distinct high molecular weight complex, which has the empirical formula, 2D-E. We have compared the compositions of lysates obtained from I125 labelled and unlabelled fibrin clots in buffer, human and animal sera, and trasylol, using conventional immunological, chromatographic and electrophoretic techniques. Both trasylol (10 KI units/ml) and various animal sera stabilised the D dimer-E

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... mplex following lysis of XL fibrin and only in buffer were free D dimer and E fragments observed. The D dimer-E complex was isolated by affinity chromatography and the expect ed polypeptide chain composition, including the crosslinked γ chain remnants, was confiimed. By combining various molar ratios of D dimer and E the expected equimolar nature of the complex was confirmed and the association sites between D dimer and E may be synonymous with the polymerization sites already shown to exist in the 0 and E domains of fibrinogen. Using I125 labelled fibrin clots, covalently linked high molecular weight complexes (up to 1 x 106Mv) were observed during in vitro lysis with plasmin. At least one of these was identified as a crosslinked γ dimer while other larger fragments may be covalently linked complexes of the "D dimer-E subunit". A hypothesis for XL fibrin lysis in vivo is proposed which complements accepted ideas on fibrin clot formation.