The expression levels of mRNAs for MDR1 (P-glycoprotein), multidrug resistance-associated proteins (MRP1, MRP2), and cytochrome P450 3A (CYP3A) in Caco-2 cells were quantitatively compared with those in human duodenal enterocytes, normal colorectal tissues, and colorectal adenocarcinomas. Caco-2 cells (passages 36-88) were kindly supplied by several laboratories in Japan. Human duodenal enterocytes were obtained from five healthy male volunteers. Normal colorectal tissues and colorectal adenocarcinomas were simultaneously obtained from seven patients with primary colorectal adenocarcinoma. MDR1, MRP1, MRP2, and CYP3A mRNA levels were determined by real-time quantitative polymerase chain reactions (PCR). Relative concentrations of mRNAs for target proteins (MDR1, MRP1, MRP2, and CYP3A) and glyceraldehyde-3-phosphate dehydrogenase in Caco-2 cells were 1.00 +/- 0.15, 1.02 +/- 0.06, 0.94 +/- 0.10, and 0.68 +/-0.60, respectively, and those in human enterocytes were about 12-, 3-, 7-, and 8000-fold higher than in the Caco-2 cells, respectively. In contrast, MDR1, MRP1, and CYP3A mRNA levels in Caco-2 cells were comparable to those in normal colorectal tissue and colorectal adenocarcinoma.