Malignantly transformed cell lines are frequently used as surrogate in vitro models for normal cell strains because they are easy to maintain. We compared the response of NHMEC strains to BP exposure with that of the malignant human mammary cell line MCF-7 in the presence and absence of chlorophyllin, a water soluble metaloporphyrin known to have anticarcinogenic properties. Cells were exposed to BP (4uM, 24h) in 4 treatment scenarios: T1-BP alone, T2-BP together with chlorophyllin, T3-BP after 24h pretreatment with chlorophyllin, T4-BP after 24h pretreatment with chlorophyllin and together with chlorophyllin. CYP1 induction was measured by reverse-transcription, real-time polymerase chain reaction (RT-PCR). Comparison of 20 NHMEC strains revealed inter-individual variation in both CYP1 induction (3-96 fold) and CYP1B1 induction (2-43 fold). BP-DNA adducts measured by (+/-)r7, t8-dihydroxy-c9,t10-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene-DNA chemiluminescence immunoassay, also showed inter-individual variation of 3-58 BP-DNA adducts/10(8) nucleotides in NHMECs exposed only to BP. Chlorophyllin treatment T4, the most effective in reducing BP dependent CYP induction and BP-DNA adducts, reduced CYP1A1 induction by 5-88% in 19/20 cell strains, and CYP1B1 induction by 10-83% in 18/20 cell strains. Similarly, DNA adduct levels were reduced by 25-87% in all the 20 cell strains exposed to treatment T4. In MCF-7 cells basal CYP1A1 and CYP1B1 RNA levels were comparable to NHMEC strains when measured by RT-PCR normalized to GAPDH. In MCF-7 cells exposed to BP, CYP1A1 was induced 114 fold and CYP1B1 by 5 fold. Formation of BP-DNA adducts in MCF-7 cells was 10 fold higher than in the highest NHMEC strain (609/10(8) nucleotides). Also, CYP induction and DNA adducts were not altered by chlorophyllin exposure. Therefore, compared to most NHMEC strains, MCF-7 cells have a highly inducible CYP1A1 and a high capacity for BP-DNA adduct formation, suggesting that these cell strains are an inappropriate surrogate model of human mammary carcinogenesis. The most important findings of this study are that chlorophyllin reduces CYP1A1 and CYP1B1 induction and BP-DNA adduct formation, in response to BP exposure. The data suggest that chlorophyllin has anticarcinogenic activity in normal human mammary tissue.