Radiation carcinogenesis almost certainly involves multiple genetic alterations. Identification of such genetic alterations would provide information to help understand better the molecular mechanism of radiation carcinogenesis. The energy released by ionizing radiation has the potential to produce DNA strand breaks, major gene deletions or rearrangements, and other base damages. Alterations of the p53 gene, a common tumour suppressor gene altered in human cancers, were examined in radiation-induced rat skin cancers. Genomic DNA from a total of 33 rat skin cancers induced by ionizing radiation was examined by Southern blot hybridization for abnormal restriction fragment patterns in the p53 gene. An abnormal p53 restriction pattern was found in one of 16 cancers induced by electron radiation and in one of nine cancers induced by neon ions. The genomic DNA from representative cancers, including the two with an abnormal restriction pattern, was further examined by polymerase chain reaction amplification and direct sequencing in exons 5-8 of the p53 gene. The results showed that one restriction fragment length polymorphism (RFLP)-positive cancer induced by electron radiation had a partial gene deletion which was defined approximately between exons 2-8, while none of the other cancers showed sequence changes. Our results indicate that the alterations in the critical binding region of the p53 gene are infrequent in rat skin cancers induced by either electron or neon ion radiation.