A technique was developed to identify and quantitate unique plasmic degradation products of crosslinked fibrin in plasma. In this method, fibrin derivatives were extracted by heat precipitation and dissolved with disulfide bond reduction, after which crosslinked .gamma.-.gamma. chain remnants were identified by SDS[sodium dodecyl sulfate]-polyacrylamide gradient gel electrophoresis and quantitated by densitometric analysis. A heterogeneous group of .gamma.-.gamma. chains with MW between 100,000-76,000 daltons was identified in lysates of crosslinked fibrin during plasmic degradation in vitro. Three stages of crosslinked fibrin degradation were arbitrarily defined based primarily on extent of degradation of these .gamma.-.gamma. polypeptide chains. As little as 20 .mu.g of crosslinked fibrin digest added to 1 ml of normal plasma could be detected by the heat-extraction-gel-electrophoresis technique, identifying .gamma.-.gamma. derivatives with MW of 96,000, 86,000, 82,000 and 76,000 daltons. Plasmic derivatives of .gamma.-.gamma. chains were not found in normal plasma, but they were identified in plasma of patients with disseminated intravascular coagulation and deep-vein thrombosis, before and in increased quantity during successful thrombolytic therapy.