The p53 tumor suppressor protein binds to both cellular and viral proteins, which influence its biological activity. One such protein is the large E1b tumor antigen 1 (E1b58kDa) from adenoviruses (Ads), which abrogates the ability of p53 to transactivate various promoters 2. This inactivation of p53 function is believed to be the mechanism by which E1b58kDa contributes to the cell transformation process 2. Although the p53–E1b58kDa complex occurs during infection 3 and is conserved among different serotypes 4, there are limited data demonstrating that it has a role in virus replication. However, loss of p53 expression occurs after adenovirus infection of human cells 4, 5 and an E1b58kDa deletion mutant (Onyx-015, also called dl 1520) selectively replicates in p53-defective cells 6, 7. These (and other) data indicate a plausible hypothesis is that loss of p53 function may be conducive to efficient adenovirus replication. However, wild-type (wt) Ad5 grows more efficiently in cells expressing a wt p53 protein 5. These studies indicate that the hypothesis may be an oversimplification. Here, we show that cells expressing wt p53, as well as p53-defective cells, allow adenovirus replication, but only cells expressing wt p53 show evidence of virus-induced cytopathic effect. This correlates with the ability of adenovirus to induce cell death. Our data indicate that p53 plays a necessary part in mediating cellular destruction to allow a productive adenovirus infection. In contrast, p53-deficient cells are less sensitive to the cytolytic effects of adenovirus and as such raise questions about the use of E1b58kDa-deficient adenoviruses in tumor therapy 6, 7.