Proliferation of the vascular endothelium occurring in brain tumours is accompanied by a proliferation of histiocytes in the peripheral part of the vessel wall. These histiocytes infiltrate the tumour tissue in a very regular pattern. Enzyme-histochemically, there are marked differences between the activities of alkaline phosphatase, 5-nucleotidase, and ATPase in the normal and proliferating blood vessels. The whole process encompasses reactive changes evoked by the destroyed perivascular sheath of astroglial foot processes and the subsequent oedema in the tumour and the surrounding parenchyma. There are often tumour areas where diminished vascular permeability is established by proliferation of perivascular connective tissue. Here the oedema has completely disappeared. A clearcut influx of monocytes from the blood into the vessel wall is seen only in the vicinity of necrotic foci; the number of histiocytes is increased and their turnover is observed in swollen macrophages. In the rest of the tumour influx of monocytes and activity of macrophages are inconspicious.