Although little is known about its metabolism, glial fibrillary acidic (GFA) protein has become widely used as a cell-specific, species-non-specific antigenic marker for normal or pathologically altered astroglia. So far there have been few investigations on GFA protein in relation to scrapie and analogous spongiform encephalopathies although there has been a need for an unequivocal method for the discrimination of different types of glia in these diseases. In the present studies, a commercially available antiserum to GFA protein incorporated in a peroxidase-antiperoxidase procedure was applied to paraffin sections of brain and spinal cord from mice affected with scrapie, avirulent Semliki forest virus and cuprizone encephalopathy, and to tissues from healthy and scrapie-affected sheep. Excellent delineation of GFA protein was obtained in astroglial cell bodies and processes and in fibrils in the glia limitans and in perivascular and subependymal sites. The method was extremely sensitive and selective. A massive increase in GFA protein in scrapie-affected mice paralleled an increase in reactive astrocytes and facilitated the construction of astroglial lesion profiles for scrapie and the other encephalopathies. In sheep, abundant GFA protein occurred in both healthy and in scrapie-affected animals and in the tissues examined the differences were not conclusive.