Extravasal fibrin deposition is frequently observed within and around tumorous tissues and has been implicated in various aspects of tumor growth. However, no adequate information has been available on the mechanism how intratumoral interstitial fibrin deposits escape a prompt elimination by the fibrinolytic system. In this study we provide immunomorphological evidence showing that fibrin deposits in lymph nodes with Hodgkin's disease are stabilized and made resistant to fibrinolysis by factor XIII (FXIII) of blood coagulation. By double immunofluorescent labelling systems fibrin deposits were simultaneously stained for alpha 2-antiplasmin (alpha 2-AP), the main physiological inhibitor of fibrinolysis and in a number of nodular areas they were also labelled for plasmin(ogen). The detection of alpha 2-antiplasmin-plasmin complex-neoantigen (alpha 2-AP-P-Neo) revealed that alpha 2-AP reacted with plasmin, i.e., alpha 2-AP covalently linked to fibrin indeed inhibited intratumoral fibrinolysis. In addition to fibrin deposits FXIII was also found in cellular elements characterized earlier as tumor associated macrophages. These cells were attached to fibrin strands suggesting that they are involved in the intratumoral fibrin formation and might be a source of fibrin stabilizing factor in the tumor stroma.