A monoclonal antibody (Mab J1-31) has been produced by using human brain homogenate as immunogen in mouse. Double-label immunofluorescence microscopy on cryostat sections of human, rabbit and rat brain, reveals staining of cells that are also stained with antiserum to glial fibrillary acidic protein (GFAP, a commonly used marker protein for astrocytes). However, there is no decrease in staining due to Mab J1-31 in sections incubated in antiserum to GFAP prior to incubation with the J1-31 ascites fluid. Immunoprecipitation of aqueous and detergent extracts of brain homogenate gives a single band at 30K by SDS-PAGE followed by autoradiography. Immunoelectron microscopy shows that the J1-31 antigen is associated with the cytoskeleton. Thus, the Mab J1-31 recognizes a new protein present in GFAP positive cells (astrocytes) in the brain.