Immunohistological techniques with specific antisera have been used to reinvestigate the problem of cellular localization of the neurospecific protein S100. In adult rat cerebellum, this protein is exclusively localized in astrocytes as shown by revealing in the same tissue secretion S100 protein and an oligodendrocyte marker, the isoenzyme II of carbonic anhydrase. The exclusive astroglial localization of S100 protein was confirmed by immuno-electron microscopy. A number of technical artefacts which may be at the origin of the claimed localization of S100 protein in neuronal cytoplasm and nuclei were also determined and described.