We have analyzed the steady-state levels of cytochrome P-450 1A1 (CYP1A1) mRNA in peripheral blood lymphocytes of 177 individuals with various CYP1A1 genotypes using a quantitative reverse transcriptase-polymerase chain reaction technique that makes use of a homologous internal standard for accurate quantitation. We found no effects of ethnicity, age, or smoking status on CYP1A1 gene expression in this population. We did see a significant 2-fold increase in the mean level of CYP1A1 mRNA in women compared with men for both Caucasians and African Americans. We observed no effect of the African American-specific polymorphism (CYP1A1(*)3) on expression of the gene. However, we found a significant 3-fold decrease in expression associated with the homozygous MspI restriction fragment length polymorphism (CYP1A1(*)2A/(*)2A).