Most carcinogens are bioactivated by cytochrome P450s (CYPs) and these enzymes within target cells are closely related to susceptibility to cancer. Since extrahepatic CYPs occur typically at much lower levels, the existence and the role of CYP in extrahepatic tissues have been difficult to assess. In this study, we modified the reverse transcriptase-polymerase chain reaction (RT-PCR) to evaluate the relative quantities of CYP 2E1 mRNA in human endometrium. Total RNA from human endometrium was reverse-transcribed and coamplif-ied by PCR in the same tube containing both primer pairs of CYP 2E1 and β-actin. The CYP 2E1 and (3-actin PCR products were 298 and 600 bp, respectively. The restriction enzyme Mbol digested these two products to the predicted size for DNA fragments, demonstrating that both PCR products were specific and CYP 2E1 mRNA exists in human endometrium. CYP 1A1 mRNA was also examined, but could not be detected clearly. Adding [α-32P]dCTP to the reaction mixture made it possible to quantify the relative yield of the CYP 2E1 PCR product in comparison with the β-actin product. The ratio of the yield of the CYP 2E1 PCR product to the β-actin PCR product could be calculated at a point of 25 cycles of amplification. This ratio and serum estradiol levels were correlated positively (r = 0.654; p < 0.05), but no relationship to serum progesterone levels was obser