&NA;Fresh xenografted human malignant astrocytoma cells migrate throughout the host rat brain. Cells from three Grade 3 human malignant astrocytomas were prelabeled withPhaseolus vulgarisleucoagglutinin (PHAL) and then xenografted into implantation pockets in rat host cerebral cortex. The human malignant astrocytoma cells in the host brain were immunocytochemically double‐labeled for the presence of PHAL, which is used as a marker for graft derived cells, and either glial fibrillary acidic protein (GFAP), a specific marker for astrocytes and astrocytoma cells, or apolipoprotein E (APOE) 7 days, 14 days, 21 days, and 1 month later. Fresh human malignant astrocytoma cells (Grade 3 and 4) contained APOE and GFAP. The xenografted cells preserved APOE and GFAP in the host. PHAL double‐labeled human malignant astrocytoma cells were found on the glia limitans along the entire circumference of the brain, in the corpus callosum, internal capsule, entopeduncular nucleus, optic tract, and median eminence. In addition, astrocytoma cells were observed in the cingulum, habenula, arcuate, and supraoptic nucleus. Astrocytoma cells entered the space of Virchow‐Robin, migrated along parenchymal blood vessels and between the ependymal and subependymal layers of the third and lateral ventricles. APOE was a consistent marker for the migrating human malignant astrocytoma cells, but not an exclusive marker of the xenografted cells, since host rat reactive astrocytes also expressed APOE. (Neurosurgery24:541‐546, 1989)