The effects of Mn2+ on the proliferation, osteogenic and adipogenic differentiation of BMSCs were evaluated by employing MTT, Delta Im, cell cycle, ALP activity, collagen production, ARS and oil red O stain assays. The results indicated that Mn2+ decreased the viability at most concentrations for 24 h, but the viability was increased with prolonging incubation time. Mn2+ at the concentrations of 1 x 10(-7) and 1 x 10(-6) mol/L decreased Delta Im in the BMSCs for 48 h. Mn2+ induced G2/M phase cell cycle arrest at tested concentrations. On day 7 and 10, the effect of Mn2+ on the osteogenic differentiation depended on concentration, but it inhibited osteogenic differentiation at all tested concentrations for 14 d. The effect of Mn2+ on the synthesis of collagen of BMSCs depended on concentration for 7 d, but Mn2+ inhibited the synthesis of collagen at all tested concentrations for 10 d. On day 14, Mn2+ inhibited the formation of mineralized matrix nodules of BMSCs at all tested concentrations, the inhibitory effect turned to be weaker with prolonging incubation time. Mn2+ promoted the adipogenic differentiation of BMSCs at all tested concentrations for 10 d, but had no effect with prolonging incubation time. These findings suggested the effects of Mn2+ on the proliferation, osteogenic differentiation and adipogenic differentiation of BMSCs are very complicated, concentration and incubation time are key factors for switching the biological effects of Mn2+ from damage to protection.