Sixteen cases of pilocytic astrocytomas with excessive Rosenthal fiber (RF) formation were examined by the indirect immunoperoxidase method for the localization of glial fibrillary acid protein (GFAP). RF never contained GFAP but they were often enclosed in plump and thickened GFAP-positive astrocytic processes. The border between the negative RF and the surrounding positive rim of cytoplasm was always sharp and without gradual transitions. The antigenic difference between RFs and glial filaments imply that glial filaments undergo a profound change in their chemical composition during their transformation into RFs. The possibility that RFs are not degradation products of glial filaments but consist of some chemically unknown substance produced by metabolically activated astrocytes cannot be excluded.