Immortalization of human keratinocytes (HKc) by human papillomavirus type 16 (HPV16) is reproducible at a high frequency, is due directly to the presence of the viral sequences in the cells, and occurs independently from the genetic characteristics of the host cells. Ten human keratinocyte strains, each derived from a different individual, were transfected with pMHPV16d and selected with G418. Eight became established lines. Two strains, which failed to grow shortly after successful G418 selection, were negative for HPV16 DNA. No lines were established following transfection of the same HKc strains with vector sequences only. The immortalized lines maintained a constant number of copies of the viral genome integrated into the cellular DNA. Each line showed a unique integration pattern of HPV16 sequences into the cellular genome, but expressed similar patterns of viral messages. Sublines able to grow in the absence of growth factors (epidermal growth factor and bovine pituitary extract), and others which became resistant to differentiation stimuli (serum and calcium) were obtained by selection in growth factor-free medium and serum-supplemented medium, respectively. The establishment of continuous cell lines is a direct consequence of the presence of viral sequences; however, because none of these lines formed tumors in nude mice, additional events must be necessary for progression of malignancy. HPV16-immortalized human keratinocyte lines can be used to investigate and identify the viral factors involved with the modification of growth and differentiation control by HPV16.