A highly sensitive,specific assay for measuring the expression of mdr 1 gene in tumor cells,based on the reverse transcription-polymerase chain reaction (RT-PCR),was established.The expression of mdr 1 gene of the drug resistant cell lines K5 62/VCR,K562/DOX,induced by vincristine and doxorubicin respectively,and the sensitive cell line K562 was examined by RT-PCR.It was shown that through 30 cycles of amplification,the mdr 1 product could be detected in the resistant cell lines K562/VCR and K562/DOX,but not in the sensitive cell line K562.The findings suggested that RT-PCR assay could be used to distinguish drug-resistunt cells from sensitive cells and might be considered as one of the important parameters in predicting the sensitivity of anticancer drugs for individual patients clinically.