In contrast to the strong association of human papillomavirus (HPV) type 16 with genital malignancies, HPV 6 has been found essentially in benign genital lesions. In these studies we show that HPV type 6 and 16 DNAs behave differently also in their ability to transform NIH 3T3 cells in cooperation with the carcinogen N-methyl-N’-nitro-N-nitrosoguanidine (MNNG). Although we could show that both HPV-6- and HPV-16-transfected genomes were integrated and expressed in NIH 3T3 cells, only the NIH 3T3 cells which contained the HPV 16 genome became fully transformed after MNNG treatment, as assessed by their ability to form colonies in soft agar and to induce tumors in nude mice. NIH 3T3 cells containing the HPV 6 genome and treated with MNNG did not show this potential. Furthermore, we could detect an increased expression of the E7 gene of HPV 16 in the carcinogen-treated cells containing the HPV 16 genome. These studies indicate that the presence of the HPV 16 genome specifically is also an essential step to in vitro cellular transformatio