In the present work, the effect of La3+ on osteoblastic differentiation of primary rat bone mar- row stromal cells (MSCs) as well as the related mechanisms are studied. Differentiation is monitored by detection of alkaline phosphatase (ALP) activity, osteocalcin secretion, the mRNA levels of Type I collagen and osteocalcin, and matrix mineralization. The results show that La3+ inhibits osteoblastic differentiation of MSCs in the early and middle stages of culture, as demonstrated by the decrease of ALP activity, osteocalcin secretion, and down-regulation of the mRNA level of osteocalcin. However, La3+ does not affect the matrix mineralization in advanced MSCs, because it up-regulates the mRNA levels of Type I collagen, and promotes ALP activity and os- teocalcin secretion in MSCs in the late stage of cul- ture. In addition, Western blot analysis exhibits that La3+ induces the phosphorylation and activation of mitogen-activated protein kinase (MAPK). Further- more, MAPK kinase inhibitor PD98059 completely blocks the inhibitory effect of La3+ on ALP activity of MSCs in the middle stage of culture. These results suggest that La3+ affects MSCs osteoblastic differen- tiation depending on differentiation stages. La3+ in- hibits osteoblastic differentiation of MSCs in the early and middle stages by a MAPK-dependent mecha- nism, but does not affect the matrix mineralization in advanced MSCs. In the present work, the effect of La3 + on osteoblastic differentiation of primary rat bone mar- row stromal cells (MSCs) as well as the related mechanisms are studied. Differentiation is monitored by detection of alkaline phosphatase (ALP) activity, osteocalcin secretion, the mRNA levels of Type I collagen and osteocalcin, and matrix mineralization. The results show that La3 + inhibits osteoblastic differentiation of MSCs in the early and middle stages of culture, as demonstrated by the decrease of ALP activity, osteocalcin secretion, and down-regulation of the mRNA level of osteocalcin. However, La3 + does not affect the matrix mineralization in advanced MSCs, because it up-regulates the mRNA levels of Type I collagen, and promotes ALP activity and os- teocalcin secretion in MSCs in the late stage of cul- ture . In addition, Western blot analysis exhibits that La3 + induces the phosphorylation and activation of mitogen-activated protein kinase (MAPK). Further- more, MAPK kinase inhibitor PD98059 completely blocks the inhibitory effect of La3 + on ALP activity of MSCs in the middle stage of culture. These results suggest that La3 + affects MSCs in osteoblastic differen- tiation depending on differentiation stages. La3 + in- hibits osteoblastic differentiation of MSCs in the early and middle stages by a MAPK-dependent mecha- nism, but does not affect the matrix mineralization in advanced MSCs.