Extravascular, intratumoral fibrin deposition is frequently observed within and around neoplastic tissue and has been implicated in various aspects of tumour growth. This is the first report on the presence and distribution of fibrinogen/fibrin in primary (14 glioblastomas) and metastatic (nine samples of lung cancer origin) human brain tumours detected by immunofluorescent techniques. All tissue samples showed specific staining for fibrinogen/fibrin. In glioblastomas fibrin deposits could be detected within and around tumour foci, while in metastatic brain tumours the tumour cell nodules were surrounded by fibrin deposits localized almost exclusively in the connective tissue compartment of tumours. Double-labelling reactions for von Willebrand factor and fibrinogen/fibrin has revealed that fibrin deposition occurred throughout the tumour stroma independently of tumour vasculature. The overlapping reactions for fibrinogen/fibrin and factor XIII subunit A, as well as the urea-insolubility of the deposits indicate the crosslinked, highly stabilized nature of fibrin both within and around tumours. Staining with Ki M7 monoclonal antibody specific for phagocytosing macrophages showed these cells to be scattered in the nonnecrotic areas in glioblastomas and to be accumulated at the interface of tumorous parenchyma and connective tissue in both primary and metastatic tumours. The close association between fibrin deposition and macrophage accumulation strongly suggests the active participation of tumour associated macrophages in the formation of stabilized intratumoral fibrin network in human brain neoplasms.