Transfection of the wild-type p53 gene into an immortalized human endothelial cell line (ECV-304) by recombinant adenoviral delivery resulted in high level expression of the wild-type p53 protein and induction of apoptosis. Increases in the number of apoptotic cells were observed within 12 h after infection of ECV-304 cells with recombinant p53 adenovirus, as deter-mined by the appearance of internucleosomal DNA fragmentation ladders and by TUNEL and electron microscopic analyses. Control cells infected with a beta-galactosidase recombinant adenovirus exhibited little or no increase in apoptosis over uninfected cells. The expression of Waf-1 and Bax gene products were in-creased substantially in apoptotic ECV-304 cells as determined by Northern blot, reverse transcription-PCR and immunoblotting analyses. Lesser increases in the expression of the PCNA gene were detected in ECV-304 cells undergoing apoptosis. Both control and apoptotic ECV-304 cells did not express detectable levels of Bcl-2 mRNA or protein in Northern blotting and immunoblotting analyses, respectively. The data suggest a role for the Bax gene product in p53-mediated apoptosis of endothelial cells.