Astrocytes in active lesions of multiple sclerosis (MS) express major histocompatibility (MHC) class II molecules, and may play an important role in the presentation of antigen to myelin-specific T cells.However, it has been postulated that astrocytes are unable to act as antigen-presenting cells (APCs) because they would lack the B7 co-stimulatory molecules to activate these T cells. By using double labeling immunofluorescence staining, we demonstrate that reactive astrocytes in chronic active plaques of multiple sclerosis express the co-stimulatory molecules B7-1 and B7-2, and hence have the necessary attributes to act as antigen-presenting cells.