We have devised a highly sensitive, specific, and quantitative assay for multidrug resistance (mdr1) mRNA expression based on the reverse transcription-polymerase chain reaction (RT-PCR). mdr1 mRNA levels were detected in 30 human primary hepatocellular carcinoma (PHC) tissue and adjacent liver tissue. Five of the patients had received chemotherapy before hepatectomy. The results show that the level of expression of mdr1 gene is higher in tumor tissue than in adjacent liver tissue. mdr1 gene is overexpressed in PHC after chemotherapy. Furthermore, mdr1 gene expression in the treated tumor adjacent liver tissue is higher than that in untreated tumor adjacent liver tissue. Our results indicated that overexpression of mdr1 gene may be responsible for the intrinsic and acquired drug resistance of PHC.