To investigate the contribution of IL-1, IL-6, and TNF to the increased osteoclastogenesis induced by estrogen deficiency, ovariectomized (ovx) mice were treated with either IL-1 receptor antagonist (IL-1ra), a competitive inhibitor of IL-1, TNF binding protein (TNFbp), an inhibitor of TNF, or the anti-IL-6 antibody (Ab) 20F3 for the first 2 wk after surgery. ovx increased the bone marrow cells secretion of IL-1 and TNF, but not IL-6, and the formation of TRAP-positive osteoclast-like multinucleated cells (MNCs) in bone marrow cultures treated with 1,25(OH)2D3. The increase in MNC formation induced by ovx was prevented by in vivo treatment with either 17 beta estradiol, IL-1ra, TNFbp, or anti-IL-6 Ab. However, the percent change in MNC formation induced by the anti-IL-6 Ab was similar in ovx and sham-operated animals, whereas IL-1ra and TNFbp were effective only in ovx mice. MNC formation was also decreased by in vitro treatment of bone marrow cultures with IL-1ra and TNFbp, but not with anti-IL-6 Ab. Ovx also increased bone resorption in vivo and in vitro, as assessed by the urinary excretion of pyridinoline cross links and the formation of resorption pits, respectively. IL-1ra, TNFbp and estrogen decreased bone resorption in vivo and in vitro whereas the anti-IL-6 Ab inhibited bone resorption in vitro but not in vivo. In conclusion, these data indicate that IL-1 and TNF play a direct role in mediating the effects of ovx on osteoclastogenesis and bone resorption. The data also suggest that IL-6 is not essential for increasing bone resorption in the early postovariectomy period.