Genistein is a natural isoflavone found in Leguminosae. The effect of genistein on osteoblastic MC3T3-E1 cells was investigated. Cells were cultured for 48 h in the presence of genistein (10(-7)-10(-5) M). Genistein (10(-6) and 10(-5) M) caused a significant elevation of protein content, alkaline phosphatase activity, and deoxyriboncleic acid (DNA) content in the cells. The effect of genistein (10-5 M) in increasing protein content, alkaline phosphatase activity and DNA content in the cells was completely prevented by the presence of cycloheximide (10(-6) M), an inhibitor of protein synthesis, suggesting that the isoflavone's effect results from a newly synthesized protein component. The effect of genistein (10(-5) M) in elevating cellular protein content and alkaline phosphatase activity was completely inhibited by the presence of trifluo-perazine (10(-5) M), staurosporine (10(-7) M) or vanadate (10(-6) M), various protein kinase inhibitors. Moreover, genistein (10(-5) M)-increased protein content and alkaline phosphatase activity in the cells was clearly abolished by the presence of anti-estrogen tamoxifen (10(-6) M). The effect of 17beta-estradiol (10(-9) M) in elevating protein and alkaline phosphatase activity in the cells was not enhanced by the presence of genistein (10(-5) M). Genistein's effect might be partly involved in estrogen action. The present study demonstrates that genistein has an anabolic effect on osteoblastic MC3T3-E1 cells.