The aim of this study was to investigate the effect of anti-CD20 monoclonal antibody (McAb) on induction of apoptosis in malignant B cell lines in vitro and to explore its possible mechanism. The human Burkitt's lymphoma cell lines (Daudi, Namalwa, Raji and Ramos cells) were cultured in vitro. The inhibitory rate of cell proliferation was detected by XTT assay, the apoptosis of cells was determined by flow cytometry. The expression of BCL-2 in human Burkitt's lymphoma cell lines (Daudi, Namalwa, Raji and Ramos cells) treated with rituximab (20 microg/ml) for 24 hours was analyzed by Western blot. The results showed that the anti-CD20 McAb had a slight anti-proliferation effect on the Daudi, Namalwa, Raji cell lines and no effect on the Ramos cell line. There is no correlation between the effect and the concentration of anti-CD20 McAb. Anti-CD20 McAb as a single agent could weakly induce the apoptosis of four cell lines. The inhibitory rate of cell proliferation ranged from 3% to 10%. Expression of BCL-2 protein was down-regulated after treated by anti-CD20 McAb for 24 hours in Raji and Namalwa cell lines. It is concluded that the anti-CD20 McAb as a monomer can slightly inhibit the proliferation of Daudi, Namalwa and Raji cell lines, the inhibition does not dependent on the treating time and the concentrations of anti-CD20 McAb. Anti-CD20 McAb as a monomer can weakly induce the apoptosis of four cell lines. Expression of BCL-2 in Raji and Namalwa cell lines is down-regulated after the cells were treated by anti-CD20 McAb for 24 hours. Down-regulation of BCL-2 expression may be one of the mechanisms enhancing the cytotoxicity of cytotoxic drugs.