The mutation frequency of pS189 shuttle vector plasmids is higher in human oral keratinocytes (NHOK) immortalized with cloned human papillomavirus-16 (HPV-16) genome than in primary normal NHOK (NHOK). To determine whether oncoproteins E6 and E7 of HPV-16 are responsible for the higher mutation frequency of the plasmids, we measured the mutation frequency in NHOK and in NHOK expressing the HPV-16 oncogenes (E6, E7, or E6 plus E7). We also measured the mutation frequency in NHOK expressing the E6 or E7 proteins of the non-oncogenic HPV-6b. The mutation frequency, either background or N-methyl-N′-nitro-N-nitrosoguanidine (MNNG)-induced, in NHOK expressing the HPV-16 oncoproteins (E6, E7, or E6 plus E7) was significantly higher than in NHOK. The HPV-16 oncogenes did not alter the nature of the MNNG-induced mutations (G : C→A : T), but increased the frequency of deletions and insertions with or without MNNG. The background or MNNG-induced mutation frequency in NHOK expressing the HPV-6b E6 or E7 proteins was the same as in NHOK. NHOK and NHOK expressing HPV6b-E6 or E7 were able to arrest the cell cycle and enhance cellular p53, p21WAF1/CIP1, and Gadd45 levels when exposed to MNNG, whereas NHOK expressing the HPV-16 E6 oncogene did not demonstrate. NHOK expressing HPV-16 E7 were able to enhance cellular p53, p21WAF1/CIP1, and Gadd45 levels, but failed to arrest cell cycle progression when exposed to MNNG. These data indicate that HPV-16 E6 and E7 oncogenes are mutagenic in human oral keratinocytes and enhance the mutagenic effect of MNNG. However, the E6 and E7 proteins of the `low risk' HPV-6b did not demonstrate such an ability.