We previously reported that p38 MAPK signaling is required for osteoclast differentiation but not osteoclast function. Here we further investigated the role of p38 MAPK in the function and differentiation of mouse bone marrow macrophages (BMMφ), common precursors of osteoclasts and dendritic cells. Lipopolysaccharide (LPS) activated the p38 MAPK signaling pathway in BMMφ by sequential phosphorylation of MAPK kinase 3/6, p38 MAPK, and activating transcription factor-2. Treatment of BMMφ with SB203580, a p38 MAPK inhibitor, suppressed LPS-induced phosphorylation of activating transcription factor-2. LPS stimulated production of IL-1β, TNFα, and IL-6 in BMMφ, and SB203580 failed to inhibit the LPS-induced cytokine production. BMMφ incorporated latex beads via phagocytosis, and SB203580 had no effect on this phagocytosis. BMMφ differentiated into dendritic cells when treated with granulocyte macrophage colony-stimulating factor together with CD40 ligand, TNFα, or LPS, and SB203580 failed to inhibit this differentiation. Thus, p38 MAPK-mediated signals are not involved in either BMMφ function or BMMφ differentiation into dendritic cells. The differentiation of BMMφ into osteoclasts in response to receptor activator of nuclear factor-κB ligand or TNFα was strongly inhibited by SB203580. These findings emphasize the crucial roles of p38 MAPK-mediated signaling in osteoclast differentiation.