To explore the effect of NF κB on bcl x gene transcription in extended drug resistance leukemia cell line HL 60/E6, drug resistant subline HL 60/E6 was derived by intermittently exposing HL 60 cells to 6 ng/ml epirubicin. Indirect immunofluorescence was used to demonstrate the location of NF κB RelA in HL 60/E6 cells. FCM analysis and RT PCR were used to detect the efficiency of liposome mediated ODN transfection and the change of bcl x L mRNA levels after 5 μmol/L phosphorothioate (PS) derivatized antisense (AS) oligodeoxynucleotide (ODN) directed to RelA was transferred into HL 60/E6 cells. The results showed that RelA remained persistently active and located at the nuclei of HL 60/E6 cells,but in the cytoplasm of HL 60 cells, the efficiency of liposome mediated ODN transfection was significantly higher than that of null ODN ( P <0.01 in 4 h, 6 h, 12 h, 24 h). Exposure of HL 60/E6 cells to 5 μmol/L AS PS ODN directed to RelA led to a maximal 40 % decline of bcl x L mRNA levels within 8 h. The inhibition rate of bcl x L mRNA was (15±1.79) %, (28±2.34) %, (40±3.47) %, (20±1.54) % in 4 h, 6 h, 8 h, 15 h, respectively, but it was less than 15 % in control group. It was concluded that NF κB was involved in regulating bcl x transcription. It was suggested that NF κB was an important factor for drug resistance in leukemia cells. To explore the effect of NF κB on bcl x gene transcription in extended drug resistance leukemia cell line HL 60 / E6, drug resistant subline HL 60 / E6 was derived by intermittently exposing HL 60 cells to 6 ng / ml epirubicin. Indirect immunofluorescence was used to demonstrate the location of NFκB RelA in HL 60 / E6 cells. FCM analysis and RT PCR were used to detect the efficiency of liposome mediated ODN transfection and the change of bcl x L mRNA levels after 5 μmol / L phosphorothioate (PS) derivatized The results showed that RelA remained persistently active and located at the nuclei of HL 60 / E6 cells, but in the cytoplasm of HL 60 cells, the efficiency of liposome mediated ODN transfection was significantly higher than that of null ODN (P <0.01 in 4 h, 6 h, 12 h, 24 h). Exposure of HL 60 / E6 cells to 5 μmol / L AS PS ODN directed to RelA led to a maximal 40% declin The inhibition rate of bcl x L mRNA was (15 ± 1.79)%, (28 ± 2.34)%, (40 ± 3.47)%, (20 ± 1.54)% in 4 h It was therefore that NF κB was involved in regulating bcl x transcription. It was suggested that NF κB was an important factor for drug resistance in leukemia cells.