Exposure to polycyclic aromatic hydrocarbons (PAHs), environmental pollutants, is associated with DNA damage and cancer induction in exposed human Populations. Studies have shown that xenobiotic DNA adduct formation is inhibited by CHL, and because this compound is a potential chemopreventive agent the underlying mechanisms are of great interest. Here, we evaluated DNA adduct formation induced by BP, and BP activating enzyme expression and protein levels in the presence or absence of CHL. For this purpose, we used human lymphoblast (MCL-5) cells that express CYP1A1, CYP1B1 and other cytochrome P450s. In cells treated for 24 hr with 4 :M BP there were 167 10-(deoxyguanosin-N2-yl)-7, 8, 9-trihydroxy-7,8,9,10-tetrahydro-BP (BPdG) adducts/10^8 nucleotides, measured by immunoassay using antiserum elicited against 7, 8-dihydroxy-9,10-epoxy-7, 8, 9, 10-tetrahydrobenzo[a]pyrene (BPDE)-DNA. Co-exposure with 4 :M BP and 4 :M CHL reduced the BPdG adduct level to 120/10^8 nucleotides, while 24 h of CHL pre-exposure reduced BPdG levels to 111/10^8 nucleotides, and in cells pre- and co-exposed with CHL had 83 BPdG adducts/10^8 nucleotides. Expression of CYP1A1, CYP1B1 and CYP3A4, determined by real time-polymerase chain reaction, was increased by 3.0-, 2.3- and 2.1-fold, respectively, in cells treated with BP alone. In cells treated with CHL alone, CYP3A4 was down-regulated and the other genes were not altered. The combination of CHL and BP in the co- and pre-exposed groups resulted in 3- to 42-fold induction of CYP1A 1 and CYP1B1, while CYP3A4 expression was unchanged. Western blot analysis revealed -2-fold increase in CYP1A1 levels in cells exposed to BP alone, BP with CHL pre- and co-exposure, and CHL alone. CYP1B1 and CYP3A4 levels were also increased in cells exposed to BP with CHL pre- and co-treatments. Thus, the lowest BPdG adducts were formed in cells having 2-fold higher levels of CYP1A1, 1B1 and 3A4 proteins, compared to the unexposed controls. The CHL-induced reduction in BP-DNA adduct formation appears to be independent of the BP-activating enzyme levels examined here, and may involve additional mechanisms. The capacity of CHL to reduce BP-DNA adducts in human Iymphoblasts suggests that CHL may be a useful chemopreventive agent in PAH-exposed humans.