The potassium salt of the carbonato oxodiperoxovanadate(V) complex, obtained by a known synthetic procedure, was thoroughly characterized by infrared, Raman, and electronic spectroscopy. The bioactivity of the complex on the cell proliferation was tested on osteoblast-like cells (MC3T3E1 osteoblastic mouse calvaria-derived cells and UMR106 rat osteosarcoma-derived cells) in culture. At low doses, the complex is more toxic for the nontransformed osteoblasts than for the tumoral ones, whereas at higher doses the deleterious effects are similar for both cell lines. This peroxo complex seems to be the most toxic compound that has so far been tested on osteoblast-like cells in culture.