Immunohistochemical studies were carried out on the new type of cerebral cortical astrocytic inclusions recently discovered in a 20-year-old patient with maldeveloped brain and micropolygyria. The inclusions appeared as eosinophilic structures (hematoxylin and eosin stain) and did not exhibit argyrophilia (modified Bielschowsky method). The inclusions were strongly stained by the antibody against S-100 protein (S 100) and to a lesser extent by the antibody to microtubule-associated protein 1B (MAP 1B). In contrast to Rosenthal fibers, the astrocytic inclusions did not react with antibodies to alpha B-crystallin, glial fibrillary acidic protein and ubiquitin. No positive reactions were obtained with antibodies against heat-shock protein 27 (HSP 27), HSP 72, actin, vimentin, desmin, cytokeratin, myelin basic protein, beta-tubulin, MAP 2, tau protein, paired helical filament, phosphorylated neurofilament protein (NFP), nonphosphorylated NFP, synaptophysin, cathepsin D, alpha 1-antichymotrypsin, alpha 1-antitrypsin and basic fibroblast growth factor. By immunoelectron microscopy, the products of the reaction with the anti-S 100 antibody appeared as heterogeneous granular deposits and with the antibody to MAP 1B they were randomly scattered throughout the astrocytic inclusions. Our results demonstrate that the immunohistochemical profile of the recently described inclusions differs from that of Rosenthal fibers. Whether the novel inclusions are involved in congenital astrocyte dysfunction and cerebral malformation remains to be established.