Here we report that the E5 proteins of human papillomavirus type 11 (HPV-11) and HPV-16 suppressed the expression of the p21(WafI/SdiI/CipI) tumor suppressor gene in NIH 3T3 cells and immortalized human keratinocytes. The promoter activity of p21 was repressed by E5 of HPV-11 and -16, suggesting that p21 gene suppression by E5 was at the transcriptional level. Using an inducible system, we demonstrated that increased induction of HPV-11 E5 in NIH 3T3 cells and keratinocytes led to increased repression of p21 promoter activity. The repression of p21 promoter activity by a series of E5 mutants was somewhat correlated with their respective transforming activities. Previously, we and other investigators showed that the E5 proteins of HPV-11 and -16 can activate the expression of c-jun. The repression of p21 gene expression might be a mechanism of oncogene-mediated growth promotion, since the expression of c-jun also led to a reduction of the levels of p21 RNA and protein in keratinocytes. This is the first demonstration that E5 proteins of HPV-11 and -16 repress p21 gene expression, and this might be one of the mechanisms by which E5 stimulates cell proliferation. In addition, this is also the first report of c-jun repression of p21.