The p53 tumor suppressor gene product interacts with the p300 transcriptional coactivator that regulates the transactivation of p53-inducible genes. The adenovirus E1A protein has been shown to bind to p300 and inhibit its function. E1A inhibits p53 transactivation and also promotes p53 accumulation by a p300-dependent mechanism. Murine double minute 2 (Mdm2) is a transcriptional target of p53 that binds to p53 and inhibits its transcriptional activity. E1A inhibited mdm2 transactivation without affecting the expression of p21(WAF1) or Bax, which resulted in high levels of p53 accumulation and apoptosis. Ectopic expression of p300 restored Mdm2 levels and inhibited p53-dependent apoptosis, as did ectopic expression of Mdm2. Thus, p300 is required for mdm2 induction by p53 and the subsequent inhibition of p53 stabilization. Inhibition of p300 by E1A results in stabilization of p53 and causes apoptosis. Moreover, E1B 19K or Bcl-2 expression in E1A-transformed cells abrogated p53-dependent apoptosis by restoring mdm2 transactivation by p53. Hence, p300 regulation of mdm2 expression controls apoptotic activity of p53, and 19K or Bcl-2 bypass E1A inhibition of p300 transactivation of Mdm2.