Several studies in humans indicate that both high and low concentrations of magnesium have harmful effects on bone metabolism and homeostasis. However, little is known about the effects of different concentrations of magnesium on bone cells. Considering that 1 mM is the physiological concentration of extracellular magnesium for cultured cells, in our experimental model we exposed osteoblast like SaOS-2 cells and normal human osteoblasts to low (0.1 mM) and high (5.0 mM) concentrations of magnesium. We found that high concentrations of magnesium markedly inhibited the deposition of mineral matrix by SaOS-2 as well as the activity of alkaline phosphatase, a marker of osteoblast differentiation. We then evaluated the differentiation of normal human osteoblasts by measuring alkaline phosphatase activity and again found a marked inhibition by high concentrations of magnesium. Nitric oxide, which is known to play a role in bone formation, does not seem to be involved. We hypothesize that high levels of magnesium might alter the intracellular concentration of various cations - among which calcium - by competing for the same transporters. We conclude that high magnesium levels impair osteoblast activity and might therefore contribute to bone disease.