【Abstract】 Objective To investigate the effects of arsenic trioxide on lymphocyte apoptosis in MRL-lpr mice with lupus nephritis, through the observation of apoptotic lymphocytes and gene expression relating to apoptosis. Methods Fourteen MRL-lpr mice were randomly assigned into experimental group or control group. Mice in experimental group were treated with arsenic trioxide and those in control group with saline. Mouse anti-dsDNA antibody titer was detected by ELISA. Apoptotic lymphocytes in spleen were assessed by flow cytometry. Caspase-3 expression was measured by RT-PCR, and bcl-2 expression by western blot. Results Anti-dsDNA antibody titer was significantly lower in experimental group than in control group. More apoptotic cells were found in spleen from mice of experimental group. Caspase-3 RNA was significantly higher in experimental group than in control group, but bcl-2 protein was less in experimental group. Conclusion Arsenic trioxide induces apoptosis of autoimmune lymphocytes in MRL-lpr mice with lupus nephritis, probably through the activation of caspase family and the inhibition of bcl-2 expression.