We studied the effect of menatetrenone, a vitamin K2 homolog, on bone resorption stimulated by interleukin-1α (IL-1α), prostaglandin E2 (PGE2), parathyroid hormone (PTH), and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. Bone-resorbing activity was assessed by measurement of calcium and hydroxyproline in the media and calvariae. IL-1α (0.1–100 U/ml), 1,25-(OH)2D3 (1010-10−7 M), PGE2 (10−9-10−6 M), and PTH (3 × 10−8-3 × 10−7 M) dose dependently increased the levels of calcium and hydroxyproline in the medium. Indomethacin (10−6 M) completely inhibited bone resorption induced by IL-1α and partially inhibited bone resorption induced by 1,25-(OH)2D3. However, indomethacin did not affect the action of PGE2 or PTH. Menatetrenone (3 × 10−6-3 × 10−5 M) inhibited the bone resorption induced by IL-1α (2 U/ml), PGE2 (10−7 M), PTH (3 × 10−7 M), and 1,25-(OH)2D3 (3 × 10−10 M) in a dose-dependent manner. Menatetrenone also inhibited the PGE2 production stimulated by IL-1α. These results indicate that menatetrenone may inhibit bone resorption through at least two different mechanisms; one possibly is an inhibitory effect on prostaglandin production.