The bax gene promoter region contains four motifs with homology to consensus p53-binding sites. In cotransfection assays using p53-deficient tumor cell lines, wild-type but not mutant p53 expression plasmids transactivated a reporter gene plasmid that utilized the bax gene promoter to drive transcription of chloramphenicol acetyltransferase. In addition, wild-type p53 transactivated reporter gene constructs containing a heterologous minimal promoter and a 39-bp region from the bax gene promoter in which the p53-binding site consensus sequences reside. Introduction of mutations into the consensus p53-binding site sequences abolished p53 responsiveness of reporter gene plasmids. Wild-type but not mutant p53 protein bound to oligonucleotides corresponding to this region of the bax promoter, based on gel retardation assays. Taken together, the results suggest that bax is a p53 primary-response gene, presumably involved in a p53-regulated pathway for induction of apoptosis.