A monoclonal antibody designated III 5H8 was shown to bind both to astrocytes and to myelin sheaths as studied with immunocytochemical techniques on brain sections and cell cultures. Binding to astrocytes was confirmed by double immunofluorescent labelling of frozen sections and cell cultures with anti-GFAP, and appeared to be sensitive to formalin treatment. Binding to myelin sheaths was confirmed by comparing sections labelled with III 5H8 with sections labelled with antibodies against axons and myelin basic protein as well as by staining of sections of hypomyelinated spinal cord with III 5H8. On immunoblots of separated white matter III 5H8 revealed two bands, while on immunoblots of purified myelin only one band was seen. The findings are discussed with respect to the function of astrocytes in white matter and shared antigenic determinants between astrocytes and oligodendrocytes.