The human carcinogen 4-aminobiphenyl (4AB) was evaluated in the MutaMouse transgenic mouse mutation assay. A single oral dose of 75 mg/kg induced 6.9-, 1.8- and 2.2-fold increases in the mutation frequency (MF) in the bladder, liver and bone marrow, respectively. Ten daily oral doses of 10 mg/kg 4AB increased the MF in the bladder, liver and bone marrow by 13.7-, 4.8- 2.4-fold the control value, respectively. The repeat dosing protocol was therefore more sensitive than the single dose protocol. Assessment of DNA samples prepared from pooled liver homogenates clearly indicated the increase in MF observed in the individual liver samples obtained from both groups of 4AB-treated mice.