Each of nine mammary carcinomas induced by a single injection of nitroso-methylurea into 50-day-old Buf/N female rats, contained a transforming H-ras-1 gene. Molecular characterization of one of the genes revealed that the twelfth codon was GAA instead of GGA of the normal allele, encoding glutamic acid in place of glycine. These results indicate that chemical carcinogenesis represents an adequate model to study the role of transforming ras genes in human neoplasia.