1. The metabolism of 3H-benzo[a]pyrene (BP), 3H-7-methylbenz[c]acridine (7MBAC) and 3H-dibenz[a, j]acridine (DBAJAC) have been studied in human liver microsomes from 13 subjects. 2. When the metabolism of these carcinogens to more polar ethyl acetate-soluble metabolites were compared, the activities towards the nitrogenous careinogens were twice that determined for BP. 3. The specific rates of formation of the three proximate carcinogens, BP-7,8-dihydrodiol, 7MBAC-3,4-dihydrodiol and DBAJAC-3,4-dihydrodiol per nmol cytochrome P-450 for 12 subjects were positively correlated. 4. These dihydrodiols constituted 5.9±0.7%mean±SEM), 57.8±2.6% and 3.0±0.4% of the total metabolites identified by cochromatography with standards, 7MBAC, DBAJAC and BP respectively.