We have examined intrinsic and external factors that influence human papillomavirus type 16 (HPV-16)-mediated immortalization of oral keratinocytes. The efficiency with which HPV can immortalize human oral keratinocytes was quantified and a considerable difference in the transfection and immortalization competence of the cells was detected. The ability of HPV-16 to immortalize oral cells appeared to be linked to the age of the culture upon transfection. The addition of dexamethasone to the transfected cultures increased the efficiency of immortalization, possibly indicating a role for a critical level of HPV gene expression in initial outgrowth of immortalized colonies. We also document in detail the changes in the oral keratinocyte induced by HPV-16 immortalization. These include alterations associated with crisis and feeder independence as well as basic changes in keratin expression and differentiation.