Recent investigations have begun to dissect the number and nature of genetic alterations associated with cancer cells. In the present study, primary human epidermal keratinocytes acquired indefinite life-span in culture but did not undergo malignant conversion in response to infection with a hybrid of adenovirus 12 and simian virus 40. Addition of Kirsten murine sarcoma virus, which contains a K-ras oncogene, to these cells induced morphological alterations associated with the acquisition of neoplastic properties. These findings demonstrate the malignant transformation of human primary epithelial cells in culture and support a multiple-step process for neoplastic conversion.