Human cytochrome P4501A1 (CYP1A1) is one of the key enzymes in the bioactivation of environmental pollutants such as benzo[a]pyrene (B[a]P) and other polycyclic aro-matic hydrocarbons. To evaluate the effect of membrane properties and distinct phospholipids on the activity of human CYP1A1 puri®ed insect cell-expressed human CYP1A1 and of human NADPH-P450 reductase were reconstituted into phospholipid vesicle membranes.