Among human papillomavirus (HPV) types, clinical association with benign vs malignant lesions correlates with the ability of the corresponding oncogenes to transform cells in vitro. However, even though HPV-11 is considered a low-risk type, we have reported previously that the E5a oncogene of HPV-11gt is capable of transforming NIH 3T3 cells in culture. In this study, we found that HPV-11gt E5a and E6 oncogenes have the ability to transform NIH 3T3 and the rat embryo fibroblast line REF 52. Cells were transfected independently with expression plasmids containing the HPV-11gt E5a or E6 oncogenes or both plasmids simultaneously to examine potential interactions. Cells containing these plasmids were phenotypically transformed and had an accelerated doubling time, loss of contact inhibition of growth, and loss of anchorage dependence for cell division. Independent cell lines containing the HPV-11gt E6 gene exhibited variable levels of phenotypic transformation that correlated with the HPV-11gt E6 gene content. The degree of phenotypic transformation could be increased by elevating the level of transcription of the E6 gene, indicating that there is a dose response effect for transformation in this system. These results suggest that increased expression of E6 may be an important factor in malignant progression of naturally occurring tumors.