Liver tumors from B6C3F1 mice induced by the potent peroxisome proliferator ciprofibrate, a hypolipidemic drug, were evaluated for the presence of transforming genes by the nude mouse tumorigenicity assay. As reported earlier, the tumors were not activated by a point mutation in codon 61 of H-ras. Two of the eight tumors examined contained a mutation in codon 13 or an H-ras gene mutated in codon 117. Screening of another 23 ciprofibrate-induced liver tumors by oligonucleotide hybridization analysis and direct DNA sequencing resulted in the identification of three tumor DNA samples with point mutations in codon 117 of the H-ras gene. In addition, another tumor sample contained a K-ras gene with a mutation in codon 61. Mutations in these codons have been seen only rarely in chemically induced liver tumors from this mouse strain. Of 15 spontaneous B6C3F1 liver tumors screened in the same manner, one exhibited a K-ras gene activated by a mutation in codon 13 and a second contained an H-ras gene activated by a mutation in codon 117. These ras gene mutations have not been reported previously from spontaneous liver tumors. The frequency and spectrum of ras oncogene mutations characterized in ciprofibrate-induced liver tumors differ significantly from the frequency and pattern identified in spontaneously occurring liver tumors. The results of this study with a limited number of samples suggest that ras protooncogene activation or activation of other protooncogenes that can be detected by the nude mouse tumorigenicity assay are not frequent events in the mechanism of carcinogenicity of the peroxisome proliferator ciprofibrate. However, the lower frequency and distinct pattern of H-ras mutations observed in these tumors disprove the assumption of promotion of spontaneous hepatocarcinogenesis by ciprofibrate.