We explored the possibility that lycopene, a carotenoid that is abundant in tomatoes, has effects on proliferation and differentiation of osteoblasts, the cells responsible for bone formation. Human osteoblast-like osteosarcoma SaOS-2 cells were cultured for 24 hours, after which varying doses of a water-dispersible microemulsion preparation of lycopene or vehicle of the same dilution were added. The cells were further cultured for 24 to 144 hours, and then the cell numbers were counted. Lycopene at 10(-6) and 10(-5) M had significant stimulatory effects on cell numbers, compared with the corresponding vehicle treatment, at all time points from 24 to 144 hours. The effects of lycopene on activity of the differentiation marker alkaline phosphatase activity in the absence or presence of dexamethasone were shown to be dependent on the stage of cell differentiation. This is the first report on the effects of lycopene on osteoblasts of human origin; the results may have important applications in the prevention of osteoporosis.