This study was purposed to investigate the reversal effect of sodium selenite on multidrug resistance in adriamycin-resistant leukemic cell line K562/ADR and its mechanisms. The cytotoxicity and the reversal effect of sodium selenite on K562/ADR cells were assayed by MTT method; the apoptosis rate of K562 and K562/ADR cells were detected by flow cytometery, the mRNA expressions of mdr1 and bcl-2 were measured by semi-quantitative reverse transcriptase polymerase chain reaction (RT-PCR). The results showed that 10 micromol/L sodium selenite significantly increased the cytotoxicity of adriamycin to K562/ADR cell and the reverse index (RI) was 2.31; the early apoptosis rate of K562 cells was elevated after treatment with 5 micromol/L Na(2)SeO(3) for 48 hours; and the medium-term and late apoptosis rate was elevated after treatment with both 5 and 10 micromol/L Na(2)SeO(3) for 48 and 72 hours. Both doses of 5 and 10 micromol/L Na(2)SeO(3) increased the early apoptosis rate of K562/ADR at 48 hours, and also increased the medium-term and late apoptosis rate after treating for 48 and 72 hours. The apoptosis rate was higher at dose of 10 micromol/L than that at 5 micromol/L, the apoptosis rate at 72 hours also was higher than that at 48 hours. The expressions of mdr1 mRNA and bcl-2 mRNA were decreased significantly by 10 micromol/L sodium selenite. It is concluded that sodium selenite can reverse the multidrug resistance in K562/ADR partially by down-regulating the expressions of mdr1 mRNA and bcl-2 mRNA, and increasing apoptosis rate of K562/ADR cells.