Erdosteine is widely used as a mucolytic agent and also has free radical scavenging and antioxidant activities. However, little is known about the mechanisms of the anti-inflammatory effect of erdosteine. We investigated the effect of erdosteine on the activation of the nuclear factor (NF)-kB/inhibitor of NFkB (IkB), and the mitogen-activated protein kinase (MAPK) and Akt pathways in the mouse macrophage cell line RAW 264.7. Cultured RAW 264.7 cells were pretreated with erdosteine and stimulated with lipopolysaccharide (LPS). In Western blotting, pretreatment with erdosteine inhibited the IkBα degradation induced in RAW 264.7 cells by LPS. LPS-induced IkB kinase (IKK) activity and NF-kB transcription were inhibited by pretreatment with erdosteine. Production of IL-6 and IL-1β was also inhibited by erdosteine pretreatment. However, erdosteine did not inhibit LPS-induced phosphorylation of Akt and MAPKs. These results suggest that the anti-inflammatory effect of erdosteine in mouse macrophages is mediated through inhibition of LPS-induced NF-kB activation.