Menaquinone-7, which is vitamin K2 (menatetrenone) with seven isoprene units, is highly contained in the fermented soybean. The effect of menaquinone-7 (MK-7) on osteoblastic bone formation was investigated. Femoral-diaphyseal and metaphyseal tissues of young male rats (4 weeks old) were cultured for 48 h in a medium containing either vehicle or MK-7 (10(-7)-10(-5) M). Calcium content, alkaline phosphatase activity, and deoxyribonuclic acid (DNA) content in the diaphyseal and metaphyseal tissues was significantly increased in the presence of MK-7 (10(-6) and 10(-5) M). The effect of MK-7 in increasing the diaphyseal and metaphyseal calcium content and alkaline phosphatase activity was completely prevented in the presence of cycloheximide (10(-6) M), an inhibitor of protein synthesis. Moreover, osteoblastic MC3T3-E1 cells after subculture were cultured for 24 h in a serum-free medium containing MK-7 (10(-7)-10(-5) M). Protein content, alkaline phophatase activity, osteocalcin and DNA content in the cells was significantly increased in the presence of MK-7 (10(-6) and 10(-5) M). The effect of MK-7 in increasing protein content, alkaline phosphatase activity, and osteocalcin production in the cells was completely blocked by cycloheximide. This study demonstrates that MK-7 has an anabolic effect on bone tissue and osteoblastic MC3T3-E1 cells in vitro, suggesting that the compound can stimulate osteoblastic bone formation.