The amplification and level of expression of four cellular oncogenes was investigated in dimethylhydrazine-induced Mouse Colon Tumor 36 of Corbett et al. [Corbett, T. H. Griswold, D. P., Jr., Roberts, B. J., Peckham, J. C., and Schabel, F. M., Jr. Cancer (Phila.), 40:2660-2680, 1977], which has been used to evaluate potential chemotherapeutic agents. A 30- to 40-fold amplification of c-myc was observed in the tumor DNA as compared to the DNA of normal colonic mucosa and liver. No changes were observed in the number of copies of c-mos, erbB, or bas. Digestion with three different restriction enzymes demonstrated that there was no major alteration in the genomic structure of c-myc in the amplified DNA. The level of c-myc RNA was 10-fold higher in both the total and polyadenylated RNA from the tumor when compared to RNA from normal colonic mucosa. Probes specific for the three myc exons were used in hybridization experiments and demonstrated that all three exons were present in the amplified DNA and in the mRNA.