Daidzein, a natural isoflavonoid found in Leguminosae, has received increasing attention because of its possible role in the prevention of osteoporosis. In the present investigation, primary osteoblastic cells isolated from newborn Wistar rats were used to investigate the effect of this isoflavonoid on osteoblasts. Daidzein (2-50 microM) increased the viability (P<0.05) of osteoblasts by about 1.4-fold. In addition, daidzein (2-100 microM) increased the alkaline phosphatase activity and osteocalcin synthesis (P<0.05) of osteoblasts by about 1.4- and 2.0-fold, respectively. Alkaline phosphatase and osteocalcin are phenotypic markers for early-stage differentiated osteoblasts and terminally differentiated osteoblasts, respectively. Our results indicated that daidzein stimulated osteoblast differentiation at various stages (from osteoprogenitors to terminally differentiated osteoblasts). We also investigated the effect of daidzein on bone morphogenetic protein (BMP) production in osteoblasts that display the mature osteoblast phenotype. The results indicated that BMP2 synthesis was elevated significantly in response to daidzein (the mRNA increased 5.0-fold, and the protein increased 7.0-fold), suggesting that some of the effects of daidzein on the cell may be mediated by the increased production of BMPs by the osteoblasts. In conclusion, daidzein has a direct stimulatory effect on bone formation in cultured osteoblastic cells in vitro, which may be mediated by increased production of BMPs in osteoblasts.