p53 is the most frequent known target for mutation in human cancer. Evidence suggests that p53 protein may be involved variously in transcription and cell cycle control, in DNA replication and in G1 checkpoint control following the cellular response to radiation induced DNA damage. p53 blocks DNA replication of the small DNA tumour virus, simian virus 40, by inhibiting unwinding of the viral origin of replication by the DNA helicase activity of the virally encoded large T antigen protein. Here we report the novel observation that human p53 protein can bind ATP and exhibits an intrinsic ATP stimulated DNA strand reassociation activity. Both activities map to the carboxyl terminal 128 amino acids of p53. Thus, in addition to any role in transcription, our results indicate that p53 is potentially capable of inhibiting mammalian replicative DNA synthesis by blocking the DNA strand separation step during replication origin recruitment. However, the ability of p53 to modulate the topological relationship between complementary nucleotide strands is also compatible with a direct role for p53 in other aspects of DNA synthesis, recombination or repair.