Astrocytes are the cells that actively participate in the process of lesion repair in the central nervous system (CNS), and reactive astrocytosis of varying degrees becomes apparent with time in any pathological condition occurring in the normally developed postnatal CNS. Ganglioside GD3 (II3a(NeuAca2-8NeuAc)-LacCer, GD3) in reactive astrocytes from autopsied patients with Creutzfeldt-Jakob disease (CJD) and old cerebral infarction was investigated immunocytochemically, using mouse IgM anti-GD3 monoclonal antibody (DSG-1). Reactive astrocytes in CJD and cerebral infarction demonstrated GD3-immunoreactivity within the cytoplasm. Normal astrocytes were negative. The present data raise the possibility that GD3 in reactive astrocytes has biological implications for the properties of the cells, such as cellular motility.