The E6 proteins of high-risk genital human papillomaviruses (HPV), such as HPV types 16 and 18, possess a conserved C-terminal PDZ-binding motif, which mediates interaction with some cellular PDZ domain proteins. The binding of E6 usually results in their ubiquitin-mediated degradation. The ability of E6 to bind to PDZ domain proteins correlates with the oncogenic potential. Using a yeast two-hybrid system, GST pull-down experiments and coimmunoprecipitations, we identified the protein tyrosine phosphatase H1 (PTPH1/PTPN3) as a novel target of the PDZ-binding motif of E6 of HPV16 and 18. PTPH1 has been suggested to function as tumour suppressor protein, since mutational analysis revealed somatic mutations in PTPH1 in a minor fraction of various human tumours. We show here that HPV16 E6 accelerated the proteasome-mediated degradation of PTPH1, which required the binding of E6 to the cellular ubiquitin ligase E6-AP and to PTPH1. The endogenous levels of PTPH1 were particularly low in HPV-positive cervical carcinoma cell lines. The reintroduction of the E2 protein into the HPV16-positive cervical carcinoma cell line SiHa, known to lead to a sharp repression of E6 expression and to induce growth suppression, resulted in an increase of the amount of PTPH1. Our data suggest that reducing the level of PTPH1 may contribute to the oncogenic activity of high-risk genital E6 proteins.