OBJECTIVE To study the influence of bone marrow mesenchymal stem cells (MSC) on the proliferation and apoptosis of allogeneic peripheral B lymphocytes in vitro. METHODS MSCs were isolated from the peripheral blood of healthy volunteer and cultured. B lymphocytes were isolated from another healthy volunteer and co-cultured with the MSCs at the B:MSC ratios of 50:1, 10:1, and 1:1 or with different concentrations of MSC supernatant (12.5%, 25%, and 50%) for 72 h, in the presence of antihuman IgM immunoglobulin goat antibodies (Anti-IgM) at a final concentration of 10 microg/mlL. The proliferation of B lymphocytes was analyzed with MTT assay. B lymphocytes and MSC were co-cultured at the ratio of 1:1 for 24 h or 48 h, with or without addition of Anti-IgM. Flow cytometric was used to detect the apoptosis of B lymphocytes. RESULTS The A value of B lymphocytes co-cultured with MSCs at different ratios were 0.521 +/- 0.093, 0.418 +/- 0.103, and 0.365 +/- 0.114 respectively. The A values of Group10:1 and Ggroup1:1 were both significantly lower than that of the control group (0.679 +/- 0.049, both P < 0.01), and the A value of Ggroup1:1 was significantly lower than that of Group10:1 (P < 0.05). The A value of B lymphocytes co-cultured with 50% MSC supernatant was 0.504 +/- 0.099, significantly lower than those of the control group and Group 12.5% (both P < 0.05). MSCs didn't induce apoptosis of B lymphocytes. The apoptosis rates of B lymphocytes co-cultured with MSCs for 24 h or 48 h, in presence or absence of Anti-IgM were 1.90% +/- 0.75%, 2.33% +/- 1.01%, 2.33% +/- 0.75%, and 1.39% +/- 0.63% respectively, without significant difference between any 2 of the four groups. CONCLUSIONS MSC and its supernatant inhibit B lymphocyte proliferation with the mechanism correlated with the MSC concentration and the MSC-secreted cytokine, but MSCs does not induce B lymphocytes apoptosis in vitro.