Papillomaviruses (PV) contain several conserved genes that may encode nonstructural proteins; however, none of these predicted gene products have been identified. Papillomavirus E6 genes are retained and expressed as RNA in PV-associated human and animal carcinomas and cell lines. This suggests that the E6 gene product may be important in the maintenance of the malignant phenotype. The E6 open reading frame of the bovine papillomavirus (BPV) genome has been identified as one of two BPV genes that can independently transform mouse cells in vitro. A polypeptide encoded by this region of BPV was produced in a bacterial expression vector and used to raise antisera. The antisera specifically immunoprecipitated the predicted 15.5-kilodalton BPV E6 protein from cells transformed by the E6 gene. The E6 protein was identified in both the nuclear and membrane fractions of these transformed cells.