Manassantin A (MSA) inhibits nitric oxide production by macrophages through the inhibition of NF-kappaB activation, but the effect of MSA on dendritic cells has not been elucidated yet. Here we investigated the inhibitory effects of MSA on immune functions of dendritic cells (DCs). MSA inhibited lipopolysaccharide (LPS)-induced phenotypic maturation of DCs, which was proved by the decreased expression of CD40, CD80, CD86, MHC-I, and MHC-II. MSA also inhibited functional maturation of DCs, that is, decreased the gene expression of IL-12, IL-1beta, TNF-alpha, and IFN-alpha/beta; enhanced antigen capture capacity of DCs; and impaired induction of allogenic T cell activation. As a mechanism of action, MSA inhibited LPS-induced activation of NF-kappaB, ERK, p38, and JNK, which played pivotal roles in toll-like receptor 4-mediated DC maturation. Collectively, these results suggested that MSA might be used for the treatment of DC-related immune diseases.