A rat cerebellar cell line, WC5, derived by transformation with Rous sarcoma virus, which is temperature-sensitive for transformation (ts-RSV), can be induced to express glial fibrillary acidic protein (GFAP). Immunofluorescence, radioimmune assay, and electron microscopy studies show that GFAP is expressed in WC5 cells grown at the nonpermissive temperature (NPT), but not at the permissive temperature (PT) for transformation. GFAP is first detectable about 3 days after incubating cells at the NPT, and reaches an apparent plateau by the seventh or eighth day. The expression of GFAP is reversible; shifting cells from the NPT to the PT causes a dramatic decrease in GFAP after 96 hr. In order to determine if the expression of GFAP is linked to the temperature-sensitive transforming activity of the viral src gene product, phenotype revertants of WC5 were established. By the criteria of morphology and growth in agar, the revertant lines, in contrast to the parent cell line WC5, were shown to exhibit a transformed phenotype at both the NPT and PT. Immunofluorescence studies on several of the revertant cell lines show that they do not express GFAP at either the PT or NPT. These findings suggest that the expression of GFAP in WC5 is linked to the expression of the src gene product. The advantage of using ts-RSV to derive neural cell lines which exhibit differentiated properties is discussed.