Dimethoxybenzidine (DMO) and dimethylbenzidine (DM) are used to synthesize dyes such as C.I. Direct Blue 15 and C.I. Acid Red 114, respectively. These commercially used dyes are metabolically degraded to DMO or DM in the intestinal tract of rodents and subsequently DMO and DM are absorbed into the blood stream. Animals were exposed to DMO, DM, or the dyes in the drinking water. Tumors obtained from control and chemical-treated animals were examined for the presence of activated oncogenes by the NIH 3T3 DNA transfection assay.