Any inﬂuence of iron in polycyclic aromatic hydrocarbon (PAH)/iron oxide mixtures on the capacity of PAHs to induce metabolizing enzymes will be one of the ways that iron oxides can affect PAH carcinogenicity. Because cytochromes P450 (CYPs) are haemoproteins, it will be of interest to investigate the possible involvement of Fe2O3 in benzo[a]pyrene (BaP)/Fe2O3 mixtures on the induction of CYP1A1 enzymes in the lung. Male Sprague-Dawley rats were instilled intratracheally with haematite (56Fe2O3 or 54Fe2O3, 3 mg), BaP (3 mg) or BaP (3 mg) coated onto haematite (56Fe2O3 or 54Fe2O3) particles (3 mg). Firstly, mRNA expressions of cyp1a1 were studied. Secondly, protein concentrations and catalytic activities (7-ethoxyresoruﬁn O-deethylase: EROD) of CYP1A1 were determined. Thirdly, 54Fe from BaP/54Fe2O3 mixtures in microsomal proteins was studied using time-of-ﬂight laser microprobe mass spectrometry (ToF-LMMS). Statistically signiﬁcant increases in mRNA expressions, protein concentrations and catalytic activities of CYP1A1 were observed in animals exposed to BaP, to BaP coated onto 56Fe2O3 particles or to BaP coated onto 54Fe2O3 particles versus controls. Both of the BaP/Fe2O3 mixtures induced higher CYP1A1 protein levels and EROD activities than BaP alone. Iron oxide particles per se did not affect mRNA levels of cyp1a1 but only enhanced BaP-mediated increases of CYP1A1 protein levels and activity. The ToF-LMMS spectrum proﬁles showed that the 54Fe/56Fe ratio in the microsomes of BaP coated onto 54Fe2O3 particle-instilled animals was 1.3 instead of the theoretical ratio (i.e. 0.063) observed in BaP coated onto 56Fe2O3 particle-instilled animals. Taken together, these novel data support the hypothesis that the Fe2O3-induced increases of the metabolic activation of BaP might rely on the property of Fe2O3 particles to enhance the BaP-induced translation rate of the cyp1a1 gene into functional haemoproteins. Copyright © 2004 John Wiley & Sons, Ltd.