Recent investigations have shown the presence of ras gene mutations and human papillomavirus (HPV) DNA in prostate carcinomas. In the present study, secondary adult human prostatic epithelial cells, upon transfection with a plasmid containing the entire HPV-18 genome, acquired an indefinite life-span in culture but did not undergo malignant conversion. Subsequent infection of these immortalized cells with the Kirsten murine sarcoma virus, which contains an activated Ki-ras oncogene, induced morphological transformation that led to the acquisition of neoplastic properties. These findings demonstrate the malignant transformation of adult human prostate epithelial cells in culture by a combination of viral oncogenes and the successive roles of HPV infection and Ki-ras activation in a multistep process responsible for prostate carcinogenesis.