We have developed a sensitive reverse transcription (RT)-polymerase chain reaction (PCR) to evaluate multidrug resistance in various tissues, using seven mdr-specific primer sets. mdr1 mRNA expression was noted in 16/197 samples. However, the apparent degree of mdr1 mRNA expression depended on the sequence of the primers employed. These findings suggest that more than two sets of primers should be used for effective RT-PCR.