Procarcinogen-activating cytochrome P450 (CYP) enzymes such as CYP1B1, CYP1A1, and CYP1A2 are considered to play an important role in chemical carcinogenesis. However, conflicting data exist with respect to CYP1B1 expression in human liver. In the present study, we measured CYP1B1 mRNA and protein expression in liver samples from 12 individuals (7 nonsmokers, 4 smokers, and 1 ex-smoker) and compared the levels to those of CYP1A1 and CYP1A2. As analyzed by real-time polymerase chain reaction, CYP1B1 mRNA was present in all samples and the inter-individual variability was 16-fold. The group mean level was 5-fold greater in smokers than nonsmokers (121 +/- 46 vs. 26 +/- 5 molecules/ng double-stranded DNA, p < 0.05). By comparison, CYP1A1 mRNA was detectable in samples from 4 of 7 nonsmokers, 3 of 4 smokers, and one ex-smoker, whereas CYP1A2 mRNA was detectable in samples from 5 nonsmokers, 4 smokers, and the ex-smoker. The mean levels of CYP1A1 and CYP1A2 mRNA were 4-fold and 9-fold greater, respectively, in smokers than nonsmokers, but the differences were not statistically significant. The inter-individual variability in CYP1A1 and CYP1A2 mRNA expression was 26-fold and 500-fold, respectively. Immunoblot analysis using several antibodies and with a larger panel (n = 27) of liver microsomes showed that CYP1A1 and CYP1B1 proteins were undetectable, whereas CYP1A2 was detectable in all samples and quantifiable in 24 of 27 samples. In summary, our novel finding indicates that CYP1B1 mRNA is expressed in human liver and the levels are increased in smokers, but the protein is undetectable.