Activation of telomerase, a ribonucleoprotein complex that synthesizes telomere repeat sequences(1-4), is linked to cell immortalization and is characteristic of most cell lines and tumours(4,5-8) Here we show that expression of the human papillomavirus type 16 (HPV-16) E6 protein activates telomerase in early-passage human keratinocytes and mammary epithelial cells. This activation was observed in cells pre-crisis, that is, before they became immortal, and occurred within one passage of retroviral infection with vectors expressing HPV-16 E6. Studies using HPV-16 E6 mutants showed that there was no correlation between the ability of the mutants to activate telomerase and their ability to target p53 for degradation, suggesting that telomerase activation by HPV-16 E6 is p53 independent. Keratinocytes expressing wild-type HPV-16 E6 have an extended lifespan, but do not become immortal(9), indicating that telomerase activation and E6-mediated degradation of p53 are insufficient for their immortalization. These results show that telomerase activation is an intrinsic, but insufficient, component of transformation by HPV.