In this study we show that E6 of human papillomavirus has the ability to deregulate the cell cycle G1/S transition. In rodent immortalized fibroblasts (NIH3T3) serum deprivation or over-expression of the cyclin-dependent kinase inhibitors, p16(INK4a) or p27(KIP1), leads to G1 cell cycle arrest. HPV16 E6 overcomes the antiproliferative signals, gaining the ability to drive serum-deprived and p16(INK4a) or p27(KIP1) over-expressing cells into S phase. E6 protein from the benign HPV type 1 displays a similar activity to HPV16 E6 to deregulate the G1/S transition. Thus, this activity appears to be conserved between E6 proteins from non-oncogenic and oncogenic HPV types. Furthermore, we show that HPV16 E6 is not able to circumvent a G1 arrest imposed by pRb mutant in which all CDK phosphorylation sites have been mutated. These data indicate that the viral protein acts upstream of pRb and its mechanism in promoting cell cycle progression is dependent on pRb phosphorylation. In summary, this study describes a novel biological function of HPV E6 and shows that the S phase entry, required for viral DNA replication, is not exclusively controlled by E7, but that E6 also is involved in this event.