The ultrastructural localization of Factor VIII-related antigen (FVIII/RAg) in the endothelial cells of malignant gliomas was studied with immunogold labelling on thin sections of LR White-embedded tissue. FVIII/RAg was localized in Weibel-Palade bodies (WPBs) as well as in cytoplasmic vacuoles and vesicles, and in extracellular spaces. Stained WPBs were found both in cells surrounding vascular channels and in several proliferating cells of solid buds and glomeruli. Our results confirm previous ultrastructural observations on non-neural tissues that FVIII/RAg is produced by endothelial cells and stored in WPBs. The presence of FVIII/RAg-stained WPBs in capillary buds and tufts indicates that at least some of the proliferating cells are endothelial in origin rather than pericytes. Thus, the contribution of endothelial cells to new vessel formation seems to be more substantial than previously reported.