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Cytochrome P450 (CYP) 1A1 located in the membrane of endoplasmic reticulum is the most important enzyme in both activation and detoxification of carcinogenic benzo[a]pyrene (BaP), in combination with microsomal epoxide hydrolase (mEH). However, it is still not clearly explained how the electron transfer is mediated by NADPH:CYP oxidoreductase (POR), another component of the microsomal enzymatic system, on CYP1A1 during BaP oxidation, and whether microsomal cytochrome b5 might influence this electron transfer.High performance liquid chromatography (HPLC) was employed for separation of BaP metabolites formed by enzymatic systems containing human CYP1A1.Human CYP1A1 expressed with POR in eukaryotic and prokaryotic expression cellular systems, in microsomes of insect cells (Supersomes) and in a membrane fraction of Escherichia coli, respectively, and these enzyme systems reconstituted with purified cytochrome b5 were utilized to study BaP oxidation. Human CYP1A1 expressed in Supersomes oxidized BaP to seven metabolites [7,8- and 9,10-dihydrodiols, 1,6-dione, 3,6-dione, 3- and 9-phenols, and a metabolite with unknown structure (Mx)], whereas this enzyme expressed in membranes of E. coli formed only the metabolites 1,6- and 3,6-diones, 3- and 9-phenols, and Mx. Addition of cytochrome b5 to CYP1A1 expressed in the eukaryotic system led to a more than 2-fold increase in BaP metabolism, but had essentially no effect on BaP oxidation by CYP1A1 expressed in E. coli.The effect of cytochrome b5 on CYP1A1 conformation and the electron transfer to this enzyme may contribute to the cytochrome b5-mediated stimulation of BaP oxidation.