A rabbit antiserum against purified bovine brain S-100b protein was produced and characterized by immunoassay and immunoblot analysis of electrophoretically resolved soluble brain proteins. Fluorescent immunohistochemistry was conducted in order to determine the cellular localization of the S-100b immunoreactivity. Double immunohistofluorescent experiments on adult rat brain tissue sections with the rabbit antiserum to S-100b and a rat monoclonal antibody to the glial fibrillary acidic protein resulted in immunolabelling of the same cells. This finding determines a strict astroglial localization of the S-100b immunoreactivity. In addition, the immunolabelling of astrocyte perikarya and processes by the S-100b immunohistochemistry is consistent with a cytoplasmic location of S-100b. In contrast, the glial fibrillary acidic protein immunohistochemistry predominantly labeled the fine fibrillary processes of the cells. The present report suggests that S-100b immunohistochemistry is of use for the specific identification and morphological description of astrocytes in the rat brain.