The human CYP1 family consists of at least three proteins, CYP1A1, CYP1A2, and CYP1B1 (1 ), CYP1A1 is absent or present at very low levels in human liver (2 , 3 ), but its expression is readily detectable in lung (4 , 5 ). By contrast, CYP1A2 is constitutively expressed in human liver (2 , 3 ) and is absent in lung (4 , 5 ). CYP1B1 is primarily an extrahepatic P450, as suggested by the findmg that CYP1B1 mRNA is present in much greater abundance in tissues such as kidney, prostate, and breast than in liver (6 ). Exposure to polycyclic aromatic hydrocarbons such as those found in cigarette smoke induces the expression of both CYP1A1 and CYP1A2 (3 , 7 ). CYP1B1 is also inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), as demonstrated by cell-culture experiments (8 , 9 ). The chemical inhibitor a-naphthoflavone, which is a potent inhibitor of CYP1A1 and CYP1A2 (10 ), also inhibits CYP1B1 and with semilar efficacy and potency (11 ). cDNA-expressed CYP1A1, CYP1A2 and CYP1B1 are each active in the oxidation of theophylline (11 ), caffeine (11 , 12 ), estradiol (13 , 14 ), benzo[a]pyrene (11 ) and 7-ethoxyresorufin (11 ). With 7-ethoxyresorufin as substrate, the rank order of specific activity (pmol/min/nmol P450) is CYP1A1 > CYP1B1 > CYP1A2 (11 ). 7-Ethoxyresorufin O -deethylation activity can be a useful and convenient catalytic monitor when conducting a comparative study using a panel of these three recombinant CYP1 enzymes (see Note 1 ). Immunoinhibition experiments with inhibitory CYP1A-selective antibodies have suggested that CYP1A2 is a major contributor to 7-ethoxyresorufin O -deethylase activity in human liver microsomes (15 , 16 ).