Cigarette smoking is the predominant risk factor for bladder cancer. Aromatic amines such as 4‐aminobiphenyl (ABP) is the major carcinogens found in tobacco smoke. Although it is generally accepted that ABP is metabolically activated via N‐hydroxylation by CYP1A2 in human liver, previous studies using Cyp1a2‐null mice indicated the involvement of other enzyme(s). Here we found that CYP2A13 can metabolically activate ABP to show genotoxicity by Umu assay. The K_m and V_max values for ABP N‐hydroxylation by recombinant CYP2A13 in E. coli were 38.5 ± 0.6 μM and 7.8 ± 0.0 pmol/min/pmol CYP, respectively. The K_m and V_max values by recombinant CYP1A2 were 9.9 ± 0.9 μM and 39.6 ± 0.9 pmol/min/pmol CYP, respectively, showing 20‐fold higher intrinsic clearance than CYP2A13. In human bladder, CYP2A13 mRNA, but not CYP1A2, is expressed at a relatively high level. Human bladder microsomes showed ABP N‐hydroxylase activity (K_m = 34.9 ± 4.7 μM and V_max = 57.5 ± 1.9 pmol/min/mg protein), although the intrinsic clearance was 5‐fold lower than that in human liver microsomes (K_m = 33.2 ± 2.0 μM and V_max = 293.9 ± 5.8 pmol/min/mg protein). The activity in human bladder microsomes was prominently inhibited by 8‐methoxypsoralen, but not by fluvoxamine, anti‐CYP1A2 or anti‐CYP2A6 antibodies. CYP2S1, which is expressed in human bladder and has relatively high amino acid identities with CYP2As, did not show detectable ABP N‐hydroxylase activity. In conclusion, although the enzyme responsible for ABP N‐hydroxylation in human bladder microsomes could not be determined, we found that CYP2A13 metabolically activates ABP. © 2006 Wiley‐Liss, Inc.