Although hypoxia (lack of oxygen in body tissues) is perhaps the most physiological inducer of the wild-type p53 gene, the mechanism of this induction is unknown. Cells may detect low oxygen levels through a haem-containing sensor protein. The hypoxic state can be mimicked by using cobalt chloride and the iron chelator desferrioxamine: like hypoxia, cobalt chloride and desferrioxamine activate hypoxia-inducible factor 1alpha (HIF-1alpha), which stimulates the transcription of several genes that are associated with hypoxia. Here we show that these treatments induce accumulation of wild-type p53 through HIF-1alpha-dependent stabilization of p53 protein. Induction of p53 does not occur in either a mutant hepatoma cell line that is unable to induce HIF-1alpha or embryonic stem cells derived from mice lacking HIF-1beta. HIF-1alpha is found in p53 immunoprecipitates from MCF7 cells that express wild-type p53 and are either hypoxic or have been exposed to desferrioxamine. Similarly, anti-haemagglutinin immunoprecipitates from lysates of normoxic PC3M cells that had been co-transfected with haemagglutinin-tagged HIF-1alpha and wild-type p53 also contain p53. Transfection of normoxic MCF7 cells with HIF-1alpha stimulates a co-transfected p53-dependent reporter plasmid and increases the amount of endogenous p53. Our results suggest that hypoxic induction of transcriptionally active wild-type p53 is achieved as a result of the stabilization of p53 by its association with HIF-1alpha.