A lambda phage clone containing a promoter region of the human Ah receptor gene was isolated. This clone spanned 13.8 kb and contained the 1st exon, the sequence of which completely matched the reported Ah receptor cDNA. Using RNase protection assay and primer extension analysis, the transcription initiation sites were determined to be 643 and 615 bp upstream of the translational initiation codon ATG. This promoter did not contain a TATA box, while multiple GC boxes were present close to the determined transcription initiation sites. Comparison of the 5' flank sequence of the human Ah receptor with its murine equivalent showed several well conserved regions, containing binding sites for known transcription factors, such as Sp1. The promoter activity was confirmed by transient transfection of chimeric constructs of the Ah receptor gene and reporter gene luciferase into hepatoma HepG2 cells.