Transfection of an immortalized cell line (AE), derived from Syrian hamster embryo cells, with human papillomavirus type 18 (HPV 18) DNA induced morphological transformation and these transformed cells were tumorigenic in nude mice. Southern blot analysis revealed that the transfected viral DNA was retained in all the cell lines tested, however, all these transformed cells contained only less than one copy per cell of viral genome. Eleven cloned cell lines were established from a tumor cell line obtained after explantation of a tumor into a nude mouse. Two lines revealed no viral sequences by both Southern blot hybridization and polymerase chain reaction, whereas the nine others contained the remaining viral sequences. These results are highly suggestive of a 'hit and run' oncogenesis by this virus.