Recent evidence suggests that the tumor-suppressor protein p53 functions as a transcriptional regulator to control cell proliferation. An interaction with p53 is required for SV40 T antigen to transform primary cells; however, the effect of T antigen binding on p53 function is not known. In order to determine if an interaction with T antigen results in loss of p53-mediated transcriptional activity, we have used vectors expressing either a p53-GAL4 fusion protein or a wild-type p53 protein in transient co-transfection assays with T-antigen expression vectors. We have demonstrated that coexpression of T antigen significantly reduces both p53-GAL4-mediated transcription from a GAL4-dependent CAT reporter and p53-mediated transcription from a consensus p53 binding site in vivo. Moreover, T antigen was able to reduce binding of p53-GAL4 to its GAL4 binding sequence in gel shift experiments in vitro. These observed activities of T antigen were all dependent upon a functional p53-binding domain. In addition, coexpression of human papillomavirus type 18 E6 protein, able to bind to p53, was able to significantly reduce p53-mediated transcription. These results suggest that an interaction of certain viral oncoproteins with p53 results in loss of transcriptional activity of p53, a function that is important for maintaining normal cell growth.