Recombinant human p53 isolated either from E. coli or from insect cells is poorly active for binding to DNA but it can be dramatically stimulated by phosphorylation, antibody binding to the carboxy-terminal negative regulatory domain, short peptides derived from this negative regulatory domain or short single strands of DNA. We report here that Xenopus p53 has a very similar behavior. Using a new set of monoclonal antibodies directed either to the amino-or the carboxyterminus of Xenopus p53, we demonstrate that the frog protein can be activated by speci®c carboxy-terminus monoclonal antibodies in order to bind to human p53 DNA response element. In addition, we report that such activation of both humans and frogs protein can also be achieved by small peptides derived from the carboxyterminus of both p53. Although, the sequence of this region is not conserved in the various p53 species, the presence of conserved basic residues indicates that such activation is charge-dependent. This is con®rmed by the ®nding that small poly-lysine peptides can activate both human and Xenopus p53. In vivo expression of Xenopus p53 indicates that this protein is able to transactivate a wide variety of human p53 response elements as long as the experiments are performed at 328C since activity at 378C, a temperature well above the natural temperature of Xenopus, is lost. Finally, we demonstrate that human mdm2 is able to down regulate the transcriptional activity of Xenopus p53.