To study the species of lanthanum (III) nitrate (La[NO3](3)) dispersed in cell media and the effect on the osteoblast differentiation of bone marrow stroma cells (BMSCs). Different La-containing precipitations were obtained by adding various concentrations of La(NO3)(3) solutions to Dulbecco's modified Eagle medium (DMEM) or DMEM with fetal bovine serum (FBS). A series of characterisation methods, including dynamic light scattering, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and protein quantification were employed to clarify the species of the different La-containing precipitations. The primary BMSCs were isolated, and the cell viability, alkaline phosphatase activity, and the formation of a mineralised nodule of BMSCs were tested when treated with different La-containing precipitations. The La(NO3)(3) solutions in DMEM could form LaPO4, which exits in the particle formation, while the La(NO3)(3) solutions in DMEM with FBS could form a La-PO4-protein compound. When treated with La(NO3)(3) solutions in DMEM, the cell viability of the BMSCs was inhibited at the concentrations of 1, 10, and 100 mu M at 1 day and 3 days. Meanwhile, the supernatant derived from the La(NO3)(3) solutions in DMEM did not affect the cell viability of the BMSCs. In addition, the precipitate derived from the La(NO3)(3) solutions in DMEM added to the complete medium inhibited the cell viability of the BMSCs at concentrations of 10 mu M and 100 mu M. When treated with La(NO3)(3) solutions in DMEM with FBS, the derived precipitate and supernatant did not affect the cell viability of the BMSCs, except for the concentration of 100 mu M La(NO3)(3). The La-PO4-protein formed from the La(NO3)(3) solutions in DMEM with FBS inhibited the osteoblast differentiation of BMSCs at the concentration of 1 mu M La(NO3)(3) (P < 0.05) but had no effect on either the osteoblast differentiation at the concentrations of 0.001 and 0.1 mu M or on the formation of a mineralised nodule at all tested concentrations of La(NO3)(3). Overall, La(NO3)(3) solutions in different cell culture media could form different La-containing compounds: La-PO4 particles (in DMEM) and a La-PO4-protein compound (in DMEM with FBS). The different La-containing compounds caused different effects on the cell viability, osteoblast differentiation, and the formation of a mineralised nodule of the BMSCs. The La-containing precipitation inhibited the osteoblast differentiation by inhibiting the expression of osteoblast-related genes and proteins, providing a theoretical basis for clinical doctors to apply phosphorus-lowering drugs such as lanthanum carbon.