To evaluate the respective action of IL-4, an anti-inflammatory cytokine, and OPG, an inhibitor of bone resorption, on the inflammatory process and the associated bone resorption in collagen-induced arthritis (CIA). After CIA induction, DBA/1 mice were treated with OPG or with IL-4 DBA/1 transfected fibroblasts or both OPG + IL-4. CIA significantly improved in IL-4 groups. OPG had no effect on arthritis clinical scores but histologic scores were reduced in OPG, IL-4, and OPG + IL-4 groups vs. nontreated CIA mice. OPG increased significantly BMD and decreased by 45% D-pyridinolin levels. Moreover association of IL-4 and OPG exerted an additive effect of BMD and resorption marker (-68%). Production of IFN-gamma in the supernatants of spleen cells was reduced in IL-4 treated mice. OPG had a moderate effect on IFN-gamma, but potentiated the inhibitory effect of IL-4. OPG and IL-4 prevent bone loss in CIA-mice model and could have additive effects on IFN-gamma secretion.