The identification of the molecular structure of fibrinogen has given a better understanding of many clinical conditions associated with abnormalities of fibrinogen. Proteolysis of fibrinogen or fibrin by its natural lytic enzyme, plasmin, leads to the formation of degradation products which are of great importance in the laboratory diagnosis of many pathological conditions. The biochemical and biological properties of these fibrinogen-fibrin degradation products and their value in the diagnosis of disseminated intravascular coagulation are presented.