A DNA sequence that confers a response to a class of rodent hepatocarcinogens termed peroxisome proliferators has been identified 2947bp upstream of the rat peroxisomal bifunctional enzyme gene. Two members of the steroid hormone receptor family, termed the peroxisome proliferator activated receptor (PPARα) and the retinoid X receptor (RXRα), co-operate to bind specifically to this sequence. Importantly, this response element (PPRE) is similar to that identified upstream of other peroxisome proliferator responsive genes such as those encoding acyl CoA oxidase and cytochrome P450 IVA6. These data therefore provide further evidence that PPARα plays an important role in mediating the action of peroxisome proliferators.