To study the expression of two different subclasses of intermediate filaments in ethylnitrosourea-induced rat cerebral gliomas, the number of cells immunopositive for each subunit protein, vimentin and astroprotein (GFAP), was quantitatively analyzed. Vimentin is a subunit protein of non-specific intermediate filaments which appear transiently in immature glial cells, while astroprotein (GFAP) is a subunit protein of glial filaments, normally expressed in mature astrocytes. Although most normal astrocytes were negative for vimentin, many tumor cells showed weak to strong immunoreaction for vimentin. The expression of vimentin was more frequent and intense in anaplastic forms of gliomas than in benign forms. Accordingly, the vimentin/GFAP ratio [the number of vimentin-positive cells divided by the number of astroprotein (GFAP)-positive cells] was increased from 0.23 to 1.86, and from 0.26 to 1.85, respectively, as oligodendrogliomas and mixed gliomas become anaplastic. The present study demonstrated that the immunohistochemical study for those two subclasses of intermediate filaments can provide important informations on the cell biological nature of glial tumors.