The in vivo formation of fibrin and its subsequent secondary fibrinolytic digestion yields a variety of crosslinked fibrin degradation products (XL/FnDP). One of these is known as D-dimer and a variety of ELISA-type commercial kits using monoclonal antibodies to D-dimer have been developed. We have investigated the possibility of establishing a standard such that these various kits might indicate the same levels of D-dimer in the same samples. We have concluded that because it seems that each individual monoclonal antibody to D-dimer targets a unique and distinct epitope in the FnDP fraction the notion of introducing a standard D-dimer which will respond uniformly to each D-dimer monoclonal antibody is not feasible. Using various monoclonal and polyclonal antibodies in conjunction with gel exclusion chromatography we have examined human plasma derived from patients with disseminated intravascular coagulation (DIC) which contained high levels of fibrin degradation products (FnDP). The data suggested that the FnDP fraction in plasma contained mostly high molecular weight (> 2 x 10(6) daltons) crosslinked fragments which contain high levels of fibrinopeptide A. Many of these crosslinked fragments crossreact with antibodies to D-dimer because they each contain D-dimer as a structural component. On the basis of this data, a novel sequence of events is proposed which may occur during the aggregation of fibrin in vivo.