2-Amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ), a food mutagen, induces forestomach tumors in CDF1 mice. We established a polymerase chain reaction (PCR)-single-strand conformation polymorphism (SSCP) analysis system to detect mutations in the mouse p53 gene exons 2-10, which encompass all five regions conserved among species, and a system to examine loss of heterozygosity (LOH) that uses newly identified polymorphisms between BALB/c and DBA mice, the parental strains of CDF1 mice. Four original forestomach tumors (one papilloma, two carcinomas, and one lymph-node metastasis) and four cell lines derived from four independent forestomach tumors were examined with the PCR-SSCP system and by polymorphism analysis. Of the four original tumors, the papilloma had a G-->A transition at the second position of codon 171, and one carcinoma had a G-->T transversion at the second position of codon 113 with loss of the wild-type allele, whereas the other two carcinomas had no detectable mutations. Of the four cell lines, two had a base substitution and LOH, and the other two had double mutations (a base substitution and a deletion). By amplification of the double mutations in a fragment, the two cell lines were shown to have four kinds of alleles, indicating induction of recombination within the p53 gene. Our results show that our PCR-SSCP analysis system is efficient for detecting p53 mutations in mouse genomic DNA and that alteration of the p53 gene plays a significant role in MeIQ-induced mouse forestomach carcinogenesis.