Glial fibrillary acidic protein (GFAP) in gliosed white matter from multiple sclerosis plaques and cerebral infarcts was examined by polyacrylamide gel electrophoresis and immunoblotting. Using a monoclonal antibody raised against human GFAP, up to 11 GFAP polypeptide bands of molecular weight 37-49 kilodaltons were identified in particulate and supernatant fractions of CNS tissue homogenates. Soluble GFAP constituted about one-quarter of the total GFAP in normal cerebral white matter. In brain lesions in which reactive astrocytes were observed microscopically, the proportion of soluble GFAP was increased, with a greater representation of the lower-molecular-weight forms. In brain chronic sclerotic plaques, almost all of the GFAP was in the particulate form. Purified particulate GFAP was susceptible to proteolysis at acid but not at neutral pH in the presence of CNS homogenates. In tissue autolysis studies, GFAP was stable in situ for periods well in excess of average CNS postmortem times.