The effects of cerium ion (Ce3+) on the proliferation, differentiation, adipocytic transdifferentiation and mineralization function of primary mouse osteoblasts (OBs) were investigated. The results indicated that Ce3+ at all concentrations (1x10(-9), 1x10, 1x10(-7), 1x10(-6), 1x10(-5), and 1x10(-4) mol/L) promoted the proliferation of osteoblasts (OBs). On day 1 and 3, Ce3+ promoted the differentiation of OBs at concentrations of 1x10(-9), 1x10(-7), and 1x10(-6) mol/L, but inhibited the differentiation of OBs at higher concentrations. On day 2, Ce3+ inhibited the differentiation of OBs at tested concentrations. On day 9 and 12, Ce3+ inhibited the adipocytic transdifferentiation of OBs at most concentrations. On day 15, Ce3+ promoted the adipocytic transdifferentiation of OBs at concentrations of 1x10(-9), 1x10(-6), 1x10(-5), and 1x10(-4) mol/L, but had no effects at other concentrations. Ce3+ inhibited the formation of mineralized matrix nodules of OBs at concentrations of 1x10(-9), 1x10(-8) and 1x10(-7) mol/L, and promoted the formation of mineralized matrix nodules of OBs at other concentrations. These findings suggested that the effects of Ce3+ on the proliferation, differentiation, adipocytic transdifferentiation and mineralization function of primary OBs depended on the concentration and culture time; moreover, they were pivotal factors for switching the biological effects of Ce3+ from toxicity to activity, from damage to protection, or from down-regulation to up-regulation.