The activation of ras proto-oncogenes by point mutation in a broad spectrum of clinical malignancies and experimentally induced tumors suggests their critical role in cancer induction. To determine whether the activation of ras proto-oncogenes by point mutation also contributes to ultraviolet B radiation (UVB)-induced skin tumorigenesis and whether this event is responsible for the different tumorigenic potentials of UVB radiation in different mouse strains, we analyzed the skin tumors induced by UVB in SKH-1 hairless and C3H mice for specific mutations in the Ha-, Ki-, and N-ras oncogenes. With the same UVB irradiation protocol, the latency period for tumor appearance was longer in C3H mice than in SKH-1 hairless mice. In addition, tumor incidence and multiplicity were also significantly higher ( P e 0.001, x2 and Wilcoxon rank sum tests) in SKH-1 hairless mice compared with C3H mice. None of the 30 skin tumor specimens (15 from each mouse strain) analyzed by polymerase chain reaction (PCR) amplification of specific codons followed by dot-blot hybridization with specific probes contained mutation in codons 13 of Ha-ras; 12,13, and 61 of Ki-ras; or 12 and 13 of N-ras. However, three of the 15 tumors in SKH-1 hairless mice showed either a G3'+ A or G3' + T transition a t second position of Ha-ras codon 12. Interestingly, one of these tumors (with a G35 + A transition) also harbored an A'82 --;r G mutation a t second position of Ha-ras codon 61. None of the tumors from C3H mice showed mutations in codons 12 or 61 of the Ha-ras oncogene. With regard to codon 61 of the N-ras oncogene, six tumors from SKH-1 hairless mice and 10 tumors from C3H mice showed an A'83 -+ T transversion. While G35 + A or G35 + T transition detected by PCR and dot-blot hybridization was confirmed by sequencing, the mutations identified similarly a t codon 61 in either the Ha-or N-ras oncogenes could not be verified by sequencing of PCR-amplified products subcloned into plasmid vectors. With the exception of the low incidence of Ha-ras oncogene mutations a t codon 12 in SKH-1 hairless mouse skin tumors induced by UVB, the striking absence of mutations in the Ha-, Ki-, and N-ras oncogenes in UVB-induced mouse skin tumors suggests that ras oncogene mutations are rare and thus are not an initiating event in photocarcinogenesis.