Multidrug resistance (MDR) of tumor cells lead by overexpression of MDR1 gene is considered as a major obstacle to successful chemotherapy. The amount of MDR1 mRNA has been correlated with the degree of drug resistance, so precise quantification of MDR1 mRNA should be useful in improving monitoring and design of chemotherapy. A competitive reverse transcription-polymerase chain reaction (RT-PCR) assay for the absolute quantification of MDR1 mRNA is described. In the first, a plasmid was constructed, which contain a MDR1 cDNA fragment. The cDNA fragment share the same MDR1 primer sequence as the cellular cDNA, but it was less 58 bp in length than cellular cDNA because of shortening by EcoRⅤ. In the second, the cDNA was transcripted in vitro as an internal standard, and then it was done RT-PCR procedure together with cellular cDNA. The two kinds of amplified cDNA fragments could be distinguished after agarose gel electrophoresis, because there were difference in their length. The concentration of cellular MDR1 mRNA was derided from the ratio between the intensities of the bands corresponding to the amplified products. The test for characterizing the MDR1 expression offers high sensitivity and specificity and is therefore of great clinical relevance.