3,2′-Dimethyl-4-aminobiphenyl and 3,2′-dimethyl-4-nitrosobiphenyl were administered by subcutaneous injection in peanut oil to 2 groups of 15 male and 15 female Syrian golden hamsters. The total dose of each compound was 5.6 mmol/kg. In the group treated with 3,2′-dimethyl-4-aminobiphenyl, 24 animals had bladder tumors. In the group treated with 3,2′-dimethyl-4-nitrosobiphenyl 25 animals had subcutaneous tumors and 2 had bladder tumors. These results indicate that 3,2′-dimethyl-4-nitrosobiphenyl is a potent locally acting carcinogen. Total doses of 99 mmol/kg of aniline, o-toluidine, nitrosobenzene, or o-nitrosotoluene administered by subcutaneous injection failed to induce tumors.