To explore the effect of NF-kappa B 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-kappa B-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-XL mRNA levels after 5 mumol/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 mumol/L AS-PS-ODN directed to RelA led to a maximal 40% decline of bcl-XL mRNA levels within 8 h. The inhibition rate of bcl-XL 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-kappa B was involved in regulating bcl-x transcription. It was suggested that NF-kappa B was an important factor for drug resistance in leukemia cells.