The level of fibrinogen (FDP) and fibrin (fDP) degradation products is one of the surprisingly few unequivocal indicators of the activity of the coagulation and fibrino (geno) lytic system. Today it is beyond doubt that, similarly as the FDP/fDP level is of unequivocal importance, traditional methods of their assessment are very equivocal. Fibrin and FDP/fDP are molecules derived from fibrinogen, which are formed from the latter as a result of a series of reactions during which structures are formed or are made available which are not present in the fibrinogen molecule. New antigenic determinants are formed, in relation to the original molecule so-called neoepitopes, neoantigens. The great predominance of monoclonal antibodies prepared in the meantime reacts with fibrinogen as well as with fibrin and their degradation products. By means of specific approaches and original methods it proved, however, possible to prepare some specific monoclonal antibodies which were successfully used in diagnostic tests in vitro; their use for detection of thrombi in vivo is tested as well as their use for an increasingly effective thrombolytic therapy after their conjugation with plasminogen activators. It may be expected that this problem will be intensely developed in the near future.