A highly sensitive method is described for measuring the in vitro metabolism of 7-ethoxycoumarin or coumarin to 7-hydroxycoumarin. This method was used to measure the O-dealkylation of 7-ethoxycoumarin and the hydroxylation of coumarin by placentas from 12 nonsmokers or from 12 women who smoked cigarettes. Although an induced level of benzo[a]pyrene (BP) hydroxylase activity was found in each placenta obtained from women who smoked cigarettes during pregnancy, 7-ethoxycoumarin O-dealkylase activity was induced (about 2 fold) only in placentas whose BP hydroxylase activity was markedly induced (50 to 100 fold). These results indicate that ethoxycoumarin dealkylase activity parallels BP hydroxylase activity in human placenta only in individuals with markedly induced BP hydroxylase activity. No detectable hydroxylation of coumarin in the 7-position was observed in placental homogenates from nonsmokers 0., from women who smoked cigarettes.Human beings are exposed to benzo[a] pyrene (BP) and other polycyclic hydrocarbon carcinogens in polluted city air/ o , 22 certain smoked and cooked foods,2, 9, 15, 16 and tobacco smoke. 3 , G, 11, 29, 30, 31 Since individuality in the metabolism of BP in