To understand astrocyte-specific transcription, we have been studying the human gfa gene. This gene encodes glial fibrillary acidic protein (GFAP), an intermediate filament protein expressed primarily in astrocytes. A survey of the gfa 5' flanking region showed it to contain several segments that contribute to expression of a chloramphenicol acetyltransferase reporter gene in transfected cells. The most active of these was the 124-bp B region, which spans bp -1612 to -1489. We have now used site-directed mutagenesis to analyze this region in greater detail, and show that the B region itself contains several important elements. The most crucial of these is a consensus AP-1 sequence, the binding site for the Fos and Jun families of transcription factors. The presence of members of both these families in the glial fibrillary acidic protein-expressing U251 cell line used for our transfection studies was verified by gel mobility-shift experiments. This is the first demonstration of the functioning of a specific transcription factor site for astrocytes, and provides a focus for future studies of glial fibrillary acidic protein regulation during development and reactive gliosis.