In the present work, the effect of La3+ on osteoblastic differentiation of primary rat bone marrow 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 osteocalcin secretion in MSCs in the late stage of culture. In addition, Western blot analysis exhibits that La3+ induces the phosphorylation and activation of mitogen-activated protein kinase (MAPK). Furthermore, 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 differentiation depending on differentiation stages. La3+ inhibits osteoblastic differentiation of MSCs in the early and middle stages by a MAPK-dependent mechanism, but does not affect the matrix mineralization in advanced MSCs.